build(linux): Set default SUNSHINE_EXECUTABLE_PATH earlier so it works

[chewi]

Description

If you don't give -DSUNSHINE_EXECUTABLE_PATH, then the systemd unit ends up with a blank ExecStart= because the default value is set too late. This sets it earlier.

Issues Fixed or Closed

Bug: gentoo/gentoo#36078

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[ReenigneArcher]

Thanks for the PR. I put your changes into #2375 (trying to get a release out sooner rather than later).