refactor(ci): move integration-test common steps into composite action

[blackboxsw]

Propsed commit message

refactor(ci): move integration-test common steps into composite action

Replace the reusable workflow .github/workflows/100-dispatch-common.yml
with a composite action .github/actions/integration-test containing
shared integration-test steps (secret assertion, checkout, LXD/SSH/
pycloudlib setup, tox run, report publish, cleanup).

This allows platform-specific secret changes and pre-flight workflow
steps such as credential file setup in subsequent workflows avoiding
pulling in platform-specific setup into the common integration testing.

Behavior is preseved. Update all existing platform workflows to now
pass in environment variables to the composite action the boundary
from workflow to composite action doesn't allow passing workflow
`secrets`.

Additionally, to run composite actions a preliminary checkout step is
required in the scheduled workflow to provide visibility to the
.github/actions dir.

Additional Context

Split out from #6969

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)