download_strategy: unlink the lock file after unlocking it

[dawidd6]

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This PR solves the issue, where .incomplete.lock files are added to the bottle (oddly only on Linux).

Test subject brew install --build-bottle neofetch:

• before patch

root@943ff26116de:/home/linuxbrew# brew ls -v neofetch
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/INSTALL_RECEIPT.json
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/LICENSE.md
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/bin/neofetch
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/.brew/neofetch.rb
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/README.md
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/.bottle/var/homebrew/locks/24736b4782b7372003a40fe3f994747e4c3a6fafbc4c5f7a8c5c5dbed58ebb0a--neofetch-7.0.0.tar.gz.incomplete.lock
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/share/man/man1/neofetch.1

• after patch

root@d81b4d2a97a0:/home/linuxbrew# brew ls -v neofetch
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/INSTALL_RECEIPT.json
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/LICENSE.md
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/bin/neofetch
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/.brew/neofetch.rb
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/README.md
/home/linuxbrew/.linuxbrew/Cellar/neofetch/7.0.0/share/man/man1/neofetch.1

[MikeMcQuaid]

Makes sense, thanks @dawidd6!