all.bash: check that minified css files are unchanged

Add a check that the minified css files correspond to the their sources.

Previously, the all.bash script generated the minified files and left
the modified files in the repo working copy. This may have helped during
development, but during CI it should be a no-op. This CL simply verifies
that. The developer can still run `devtools/cmd/static` manually.

A check like this should be standard procedure for all binary or
incrutable files that appear in CLs.

Change-Id: I47d34e5204637ff38113b91ff46c595b9f42f4ee
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/585476
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

all.bash

@@ -181,6 +181,11 @@ check_script_hashes() {
 # run_build_static builds JavaScript output from TypeScript source files.
 run_build_static() {
   runcmd $GO run ./devtools/cmd/static
+  files=$(modified_files)
+  echo "mod: $files"
+  if [[ $(filter "$files" 'static/**/*.min*') != '' ]]; then
+    err "minimized CSS files are not consistent with unminimized ones; run ./devtools/cmd/static to regenerate them"
+  fi
 }
 
 run_npm() {