all commands: use /tmp/bazel-zig-cc (#6)

Different Bazel instances can share the zig cache all right.

bin/mod-tidy

@@ -4,7 +4,7 @@
 # Licensed under the Apache License, Version 2.0
 set -xeu
 
-cd "$(git rev-parse --show-toplevel)/"
+cd "$(git rev-parse --show-toplevel)"
 echo "--- go mod tidy"
 tools/bazel run @go_sdk//:bin/go -- mod tidy