avoid `--gc:arc` issue in validator key caching (#6203)
The current implementation of the validator key cache as introduced in
#5883 leads to issues when compiling with `--gc:arc`. Namely, the assert
in `injectdestructors.nim` > `destructiveMoveVar` is triggered:
```nim
assert n.kind != nkSym or not hasDestructor(c, n.sym.typ)
```
`cached == nkSym`, and `n.sym.typ == ref HashedValidatorPubKeyItem` with
`hasDestructor(c, n.sym.typ) == true`.
Inlining the `addr ...[]` avoids the problem and allows `--gc:arc`
compilation as part of LC wasm demo project.
Compilation command:
```sh
nim c \
-d:disable_libbacktrace \
-d:disableMarchNative \
-d:disableLTO \
-d:emscripten \
-d:release \
-d:useGcAssert \
-d:useSysAssert \
--debuginfo:off \
--nimcache:nimcache \
--os:linux \
--cpu:wasm32 \
--cc:clang \
--clang.exe:emcc \
--clang.linkerexe:emcc \
--gc:arc \
--exceptions:goto \
--define:noSignalHandler \
--define:danger \
--panics:on \
--passC:-fpic \
--passL:-Os \
--passL:-fpic \
--passC:'-pthread' \
--passL:'-pthread' \
--passC:'-sASSERTIONS' \
--passL:'-sASSERTIONS' \
--passC:'-sINITIAL_MEMORY=256MB' \
--passL:'-sINITIAL_MEMORY=256MB' \
--passC:'-sSTACK_SIZE=128MB' \
--passL:'-sSTACK_SIZE=128MB' \
--passC:'-sUSE_PTHREADS=1' \
--passL:'-sUSE_PTHREADS=1' \
--passC:'-sPTHREAD_POOL_SIZE_STRICT=0' \
--passL:'-sPTHREAD_POOL_SIZE_STRICT=0' \
--passL:'-sEXPORTED_FUNCTIONS="[_free, _malloc, _NimMain, _ETHRandomNumberCreate, _ETHConsensusConfigCreateFromYaml, _ETHConsensusConfigGetConsensusVersionAtEpoch, _ETHBeaconStateCreateFromSsz, _ETHBeaconStateDestroy, _ETHBeaconStateCopyGenesisValidatorsRoot, _ETHRootDestroy, _ETHForkDigestsCreateFromState, _ETHBeaconClockCreateFromState, _ETHBeaconClockGetSlot, _ETHLightClientStoreCreateFromBootstrap, _ETHLightClientStoreDestroy, _kETHLcSyncKind_UpdatesByRange, _kETHLcSyncKind_FinalityUpdate, _kETHLcSyncKind_OptimisticUpdate, _ETHLightClientStoreGetNextSyncTask, _ETHLightClientStoreGetMillisecondsToNextSyncTask, _ETHLightClientStoreProcessUpdatesByRange, _ETHLightClientStoreProcessFinalityUpdate, _ETHLightClientStoreProcessOptimisticUpdate, _ETHLightClientStoreGetFinalizedHeader, _ETHLightClientStoreIsNextSyncCommitteeKnown, _ETHLightClientStoreGetOptimisticHeader, _ETHLightClientStoreGetSafetyThreshold, _ETHLightClientHeaderCreateCopy, _ETHLightClientHeaderDestroy, _ETHLightClientHeaderCopyBeaconRoot, _ETHLightClientHeaderGetBeacon, _ETHBeaconBlockHeaderGetSlot, _ETHBeaconBlockHeaderGetProposerIndex, _ETHBeaconBlockHeaderGetParentRoot, _ETHBeaconBlockHeaderGetStateRoot, _ETHBeaconBlockHeaderGetBodyRoot, _ETHLightClientHeaderCopyExecutionHash, _ETHLightClientHeaderGetExecution, _ETHExecutionPayloadHeaderGetParentHash, _ETHExecutionPayloadHeaderGetFeeRecipient, _ETHExecutionPayloadHeaderGetStateRoot, _ETHExecutionPayloadHeaderGetReceiptsRoot, _ETHExecutionPayloadHeaderGetLogsBloom, _ETHExecutionPayloadHeaderGetPrevRandao, _ETHExecutionPayloadHeaderGetBlockNumber, _ETHExecutionPayloadHeaderGetGasLimit, _ETHExecutionPayloadHeaderGetGasUsed, _ETHExecutionPayloadHeaderGetTimestamp, _ETHExecutionPayloadHeaderGetExtraDataBytes, _ETHExecutionPayloadHeaderGetBaseFeePerGas, _ETHExecutionPayloadHeaderGetBlobGasUsed, _ETHExecutionPayloadHeaderGetExcessBlobGas, _ETHExecutionBlockHeaderCreateFromJson, _ETHExecutionBlockHeaderDestroy, _ETHExecutionBlockHeaderGetTransactionsRoot, _ETHExecutionBlockHeaderGetWithdrawalsRoot, _ETHTransactionsCreateFromJson, _ETHTransactionsDestroy, _ETHTransactionsGetCount, _ETHTransactionsGet, _ETHTransactionGetHash, _ETHTransactionGetFrom, _ETHTransactionGetNonce, _ETHTransactionGetMaxPriorityFeePerGas, _ETHTransactionGetMaxFeePerGas, _ETHTransactionGetGas, _ETHTransactionIsCreatingContract, _ETHTransactionGetTo, _ETHTransactionGetValue, _ETHTransactionGetInputBytes, _ETHTransactionGetBytes, _ETHTransactionGetEip6493Root, _ETHTransactionGetEip6493Bytes, _ETHTransactionGetNumEip6493SnappyBytes, _ETHReceiptsCreateFromJson, _ETHReceiptsDestroy, _ETHReceiptsGet, _ETHReceiptHasStatus, _ETHReceiptGetBytes, _ETHReceiptGetEip6493Bytes, _ETHReceiptGetNumEip6493SnappyBytes]"' \
--passL:'-sEXPORTED_RUNTIME_METHODS="[lengthBytesUTF8, stringToNewUTF8]"' \
--passL:'-Wl,--no-entry' \
--noMain:on \
--passL:'-o libnimbus_lc.js' \
nimbus-eth2/beacon_chain/libnimbus_lc/libnimbus_lc.nim
```
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