service: use default.target on Linux

[SMillerDev]

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Should fix https://github.com/Homebrew/homebrew-services/issues/508

[BrewTestBot]

Review period will end on 2022-09-22 at 13:21:19 UTC.

[BrewTestBot]

Review period ended.

[dawidd6]

I allowed myself to fix the tests to get this PR merged faster as the issue just hit me.

[dawidd6]

Tested locally, works alright but needs to first stop the service, reinstall a formula and start the service back up.

[dawidd6]

Those test failures seem to be unrelated?

[enernico]

I can confirm that this works perfectly fine for me, great job and many thanks! :pinata: