More CI (#3)

- ci only: use a separate zig cache dir for each run
- use `tools/bazel` everywhere.
- remove arm64 tests. They don't give enough value to be worth the
  brittle environment.
- remove windows execution, except for launcher. Ditto. I will probably
  bring them back.

release

@@ -27,12 +27,13 @@ last_tag=$(git -c 'versionsort.suffix=-' tag -l --sort=v:refname | tail -1)
     echo Changelog since "$last_tag":
     git log --pretty=format:"- [%an] %s" "$last_tag"..HEAD | \
         grep -v "Update release notes for $last_tag"
-} | git tag "${sign[@]}" -F - "$1"
+} | git tag "${sign[@]+"${sign[@]}"}" -F - "$1"
 
 shasum=$(git archive --prefix="bazel-zig-cc-$1/" --format=tar "$1" | \
     gzip -n | sha256sum | cut -f1 -d" ")
 
-./relnotes.awk -v tag="$1" -v sha256sum="$shasum" README.md | sponge README.md
+./relnotes.awk -v tag="$1" -v sha256sum="$shasum" README.md > .README.md
+mv .README.md README.md
 
 git add README.md
 git commit -m "Update release notes for $1"