[build] Silence gcc warnings on coverage bot.

GCC warnings are already treated as failures on other bots. They don't
add value on the coverage bot and only block creating reports.

Bug: v8:13005
Change-Id: I2d55288a4790e8cd508cdb8340265c696e44b671
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3747864Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81552}

BUILD.gn

@@ -1525,6 +1525,9 @@ config("v8_gcov_coverage_cflags") {
   cflags = [
     "-fprofile-arcs",
     "-ftest-coverage",
+    # We already block on gcc warnings on other bots. Let's not block here to
+    # always generate coverage reports.
+    "-Wno-error",
   ]
 }