ci: workaround for nim 1.6 and nim devel

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
         include:
           - target:
               os: linux
-            builder: ubuntu-18.04
+            builder: ubuntu-20.04
             shell: bash
           - target:
               os: macos
@@ -98,7 +98,7 @@ jobs:
       - name: Restore Nim DLLs dependencies (Windows) from cache
         if: runner.os == 'Windows'
         id: windows-dlls-cache
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           path: external/dlls-${{ matrix.target.cpu }}
           key: 'dlls-${{ matrix.target.cpu }}'
@@ -158,5 +158,6 @@ jobs:
           nim --version
           nimble --version
           nimble install -y --depsOnly
+          rm -f nimble.lock
           env NIMLANG=c nimble test
           env NIMLANG=cpp nimble test