.cirrus.yml: Add test_dub_package_script instruction

[ibuclaw]

This stopped being tested when the semaphoreci.sh script was removed in a previous PR.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

Bugzilla references

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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#13781"

[dkorpel]

It fails on certain platforms:

FreeBSD/Ubuntu (bootstrap):

Running pre-generate commands for dmd:lexer...
/bin/sh: : Permission denied

MacOS:

Failed to invoke the compiler 
/private/var/folders/tn/f_9sf1xx5t14qm_6f83q3b840000gn/T/cirrus-ci-build/generated/darwin/release/64/dmd 
to determine the build platform: /bin/sh: 
/private/var/folders/tn/f_9sf1xx5t14qm_6f83q3b840000gn/T/cirrus-ci-build/generated/darwin/release/64/dmd: 
No such file or directory

Can you fix this @ibuclaw ?