CI: Migrate Win64-DMD-bootstrap job from Azure Pipelines to GHA

[kinke]

Pros:

• 4 virtual CPU cores instead of 2 for the CI runners, and ~doubled RAM presumably too, so ~halved CI job runtime, or even better:

  • ~15 mins here: https://github.com/dlang/dmd/actions/runs/13777446959/job/38529427312?pr=20973
  • ~50 mins for the latest master job: https://dev.azure.com/dlanguage/dmd/_build/results?buildId=46313&view=logs&jobId=47916d67-4a7b-51e4-ebf4-f45083e4cd9e&j=47916d67-4a7b-51e4-ebf4-f45083e4cd9e (and otherwise 30+ mins for other comparable Azure jobs)

  There are warnings like Free memory is lower than 5%; Currently used: 97.07% in the output (for other Azure jobs too), so seems like the Azure Pipelines runners run out of memory and start awfully slow swapping.

• Reduced dependency on Azure Pipelines and their separate set of CI scripts

Cons:

• One more GHA job - with the default concurrency limit of 20 jobs

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20973"

[kinke]

Oh - this definitely accounted for longer runtimes specifically for the bootstrap job - it ran all default permutations, so incl. -release, and so doubling the number of compiler test permutations (16 vs. 8).

This is otherwise only enabled for Linux CI jobs:

dmd/ci/run.sh

Lines 119 to 124 in 9a8302a

	# default to testing fewer compiler argument permutations to reduce CI load
	if [ "$FULL_BUILD" == "true" ] && [ "$OS_NAME" == "linux" ]; then
	local args=() # use all default ARGS
	else
	local args=(ARGS="-O -inline -release")
	fi