CI: Use host dmd to compile dshell tests

[MoonlightSentinel]

This ensures that a bug / breakage in the tested compiler doesn't prevent the actual test execution (because run.d aborts when the test tools fail to build).

The only drawback is that this limits the tests to the bootstrap compiler version.

[dlang-bot]

Thanks for your pull request and interest in making D better, @MoonlightSentinel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon. Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

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Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

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#13981"

[RazvanN7]

@MoonlightSentinel is this ready for review?