github actions: purge the cache

This is done by changing the cache key.

go get tools currently fail with:
  # cd /home/runner/go/src/golang.org/x/tools; git pull --ff-only
  From https://go.googlesource.com/tools
  + 63754364...8b4aab62 master             -> origin/master  (forced update)
    ab9934f0..f2d1c246  gopls-release-branch.0.6 -> origin/gopls-release-branch.0.6
  * [new branch]        internal-branch.issue43628 -> origin/internal-branch.issue43628
  * [new tag]           gopls/v0.6.2       -> gopls/v0.6.2
  * [new tag]           gopls/v0.6.3-pre.1 -> gopls/v0.6.3-pre.1
  fatal: Not possible to fast-forward, aborting.
  package golang.org/x/tools/go/analysis: exit status 128

See https://github.com/periph/devices/pull/2/checks?check_run_id=1693498363

.github/workflows/test.yml

@@ -53,7 +53,7 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/go
-        key: "${{runner.os}}-gopkg-${{hashFiles('go.sum', '.github/workflows/*.yml')}}"
+        key: "${{runner.os}}-gopkg-${{hashFiles('go.sum', '.github/workflows/*.yml')}}-v2"
 
       # Fetch the tools before checking out, so they don't modify go.mod/go.sum.
     - name: 'go get necessary tools'