Add gen_all --check-only to CI

Adds --check-only to tools/gen_all. This just passes down
the arg to the gen scripts. Adds the invocation to all CI bots.
Also does minor cleanups to the CI scripts.

Bug: 123498112
Change-Id: I3215b88a18f8e5b6ea6dd2e731134f96e64e9e8e

tools/gen_binary_descriptors

diff --git a/tools/gen_binary_descriptors b/tools/gen_binary_descriptors
index 1a9d633..1d26587 100755
--- a/tools/gen_binary_descriptors
+++ b/tools/gen_binary_descriptors
@@ -46,12 +46,9 @@
 
 
 def find_protoc():
-  for root, dirs, files in os.walk(ROOT_DIR):
+  for root, _, files in os.walk(os.path.join(ROOT_DIR, 'out')):
     if 'protoc' in files:
       return os.path.join(root, 'protoc')
-    for name in ('src', 'buildtools'):
-      if name in dirs:
-        dirs.remove(name)
   return None