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.

.gitignore

@@ -6,3 +6,5 @@
 /bazel-out
 /bazel-testlogs
 /bazel-x
+
+.custom.ci.bazelrc