bump-formula-pr: Determine Linux-only formulae with depends_on :linux

- We used to use `# tag "linux"` in Homebrew/linuxbrew-core for the ~50
  Linux-only formulae. As of
  6578a4aa86,
  we're using `depends_on :linux` to have a consistent syntax between
  Linux and macOS.
- Therefore, we have to change the search string for Linux-only formulae
  that determines whether or not `brew bump-formula-pr` deletes the bottle
  line.

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -259,7 +259,7 @@ module Homebrew
     # When bumping a linux-only formula,
     # one needs to also delete the sha256 linux bottle line.
     # That's because of running test-bot with --keep-old option in linuxbrew-core.
-    if formula.path.read.include?('# tag "linux"')
+    if formula.path.read.include?("depends_on :linux")
       replacement_pairs << [
         /^    sha256 ".+" => :x86_64_linux\n/m,
         "\\2",