Update check_deps, make reusing scripts easier (#1187)

I went down this route because I figured the installer binary should be in `check_deps`. The result:

- Add thin wrapper scripts for `buildozer` and `bazel`, similar to the `buildifier` wrapper that's already there.
  - These are mainly to avoid deduplicating logic, and dodging intricacies of Python imports that I don't understand well through instead subprocessing.
- Fix update_roots.py (it was missing a `.parent`, must've been relocated without a re-run at some point)
- Fix fuzzer proto code to be `testonly`
- Update the `check_deps` targets

scripts/scripts_utils.py

@@ -158,10 +158,9 @@ def locate_bazel() -> str:
     if bazel:
         return bazel
 
-    if shutil.which("bazelisk"):
-        return "bazelisk"
-
-    if shutil.which("bazel"):
-        return "bazel"
+    for cmd in ("bazelisk", "bazel"):
+        target = shutil.which(cmd)
+        if target:
+            return target
 
     exit("Unable to run Bazel")