fix(build): Don't depend on execSync running bash (#6262)

For some reason on Github CI servers execSync uses /bin/sh, which
is (on Ubuntu) dash rather than bash, and does not understand
the pipefail option.

So remove the grep pipe on stderr and just discard all error output
at all.

This is not ideal as errors in test deps will go unreported AND
not even cause test failure, but it's not clear that it's worth
investing more time to fix this at the moment.

scripts/gulpfiles/build_tasks.js

@@ -343,18 +343,14 @@ function buildDeps(done) {
 
   const args = roots.map(root => `--root '${root}' `).join('');
   execSync(
-      `set -o pipefail; \
-       (closure-make-deps ${args} >'${DEPS_FILE}') 2>&1 \
-       | (grep -v '^WARNING in' ; true)`,
+      `closure-make-deps ${args} 2>/dev/null >'${DEPS_FILE}'`,
       {stdio: 'inherit'});
 
   // Use grep to filter out the entries that are already in deps.js.
   const testArgs = testRoots.map(root => `--root '${root}' `).join('');
   execSync(
-      `set -o pipefail; \
-       (closure-make-deps ${testArgs} | grep 'tests/mocha' \
-            > '${TEST_DEPS_FILE}') 2>&1 \
-       | (grep -v '^WARNING in' ; true)`,
+      `closure-make-deps ${testArgs} 2>/dev/null \
+           | grep 'tests/mocha' > '${TEST_DEPS_FILE}'`,
       {stdio: 'inherit'});
   done();
 }