Install standalone build dependencies in Ubuntu test jobs #1383

The Ubuntu strict job fails in the existing beta installer acceptance test because the test clones main and builds the standalone runner, but the test workflow does not install shfmt.

The failure is shfmt is required to build the standalone binary, before any assertions in test_install_downloads_the_non_stable_beta_version run. It blocks #1382. The same failure reproduces on baseline commit 911b69ab in GNU Bash 5.3.9 without shfmt, so it is independent of the provider result fix.

Failing CI job: https://github.com/TypedDevs/bashunit/actions/runs/37046235541/job/110968247318

The build verification workflow already installs the pinned standalone optimizer. The Ubuntu test matrix and localized Ubuntu jobs also run the installer acceptance tests and need the build dependencies available.

Acceptance criteria:

• Install the required standalone build dependencies before Linux jobs that run the beta installer acceptance test.
• Keep the optimizer version consistent with the build verification workflow.
• The beta installer acceptance test passes in strict mode without skipping it for a missing dependency.
• The full Ubuntu strict suite passes.