bump-formula-pr: Only delete Linux bottle line if it exists

[issyl0]

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• We have some Linux-only formulae that don't have bottles.
• Previously, bumping a Linux-only formula that didn't have a bottle
  line - eg, adoptopenjdk which is bottle :unneeded - would fail:

Error: inreplace failed
/app/linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/adoptopenjdk.rb:

expected replacement of /^    sha256 ".+" => :x86_64_linux\n/m with "\2"

Fixes https://github.com/Homebrew/linuxbrew-core/issues/19633