ci: Don't trigger for main pushes & run tests in release

[tomkerkhove]

• Don't run CI when merged to main (as PR already did the same)
• Run everything in Release mode, similar to release

It's good to note that tests always run in Debug anyway, as this is defined in Fake like this:

SpanNetty/build.fsx

Lines 259 to 261 in a4ba4e6

	match (hasTeamCity) with
	| true -> (sprintf "test -c Debug --no-build --logger:trx --logger:\"console;verbosity=normal\" --framework %s -- RunConfiguration.TargetPlatform=x64 --results-directory \"%s\" -- -parallel none -teamcity" testNetFrameworkVersion outputTests)
	| false -> (sprintf "test -c Debug --no-build --logger:trx --logger:\"console;verbosity=normal\" --framework %s -- RunConfiguration.TargetPlatform=x64 --results-directory \"%s\" -- -parallel none" testNetFrameworkVersion outputTests)

[tomkerkhove]

I believe the primary difference we've noticed is that release pipeline was still on Incrementalist which ran tests differently and those were triggered in our CI as well.

Examples (release vs latest CI from main):

After checking, all failed tests are passing in CI so should be fine with this alignment. (although I can only test on release)