load: pass context arguments to subprocesses (#724)

Summary:
This fixes a bug where, if the `SOURCECRED_DIRECTORY` environment
variable is set to `foo` but the `-d bar` flag is passed, then the
repository registry will be written under `foo` but the plugin data will
be loaded under `bar`.

Test Plan:

```
$ rm -rf /tmp/good /tmp/bad
$ SOURCECRED_DIRECTORY=/tmp/bad >/dev/null \
> node bin/sourcecred.js load sourcecred/example-github -d /tmp/good
$ [ -d /tmp/bad ]; echo $?
$ find /tmp/good
/tmp/good
/tmp/good/cache
/tmp/good/cache/sourcecred
/tmp/good/cache/sourcecred/example-github
/tmp/good/cache/sourcecred/example-github/github
/tmp/good/cache/sourcecred/example-github/git
/tmp/good/repositoryRegistry.json
/tmp/good/data
/tmp/good/data/sourcecred
/tmp/good/data/sourcecred/example-github
/tmp/good/data/sourcecred/example-github/github
/tmp/good/data/sourcecred/example-github/github/view.json
/tmp/good/data/sourcecred/example-github/git
/tmp/good/data/sourcecred/example-github/git/graph.json
```

wchargin-branch: load-pass-context

src/cli/commands/load.js

@@ -131,6 +131,10 @@ function loadDefaultPlugins({
         githubToken,
         "--output",
         repoToString(outputRepo),
+        "-d",
+        basedir,
+        "--max-old-space-size",
+        maxOldSpaceSize,
       ],
       deps: [],
     })),