fix(ci): keep Android ccache key stable through build

[andrescera]

What

Compute the Android source-aware ccache key once from the clean post-checkout tree and reuse that immutable value for both restore and save.

Why

Trusted run 29296248309 exposed save-key drift: both attempts requested the same clean-tree key, but build-android generated files beneath the hashed scripts/** tree before the save expression was evaluated. Each attempt therefore saved a different key and could only restore an older prefix match.

How to verify

• Run Actionlint and parse all workflow YAML.
• Run the workflow trigger checker and codespell.
• Run srt-ccache-completion-20260713.assert.sh; the merged baseline fails the four key-stability assertions and this branch passes them.
• In hosted Android CI, confirm the restore and save steps report the same key. On the subsequent trusted warm run, confirm an exact-key restore and material ccache hits.

Risks

Low. The NDK r23 build command, source-hash inputs, 200 MB bound, pull-request restore-only policy, and all source/test/release behavior remain unchanged.