build: gate debug prefix map to GitHub Actions

[scarf005]

Purpose of change (The Why)

Regression from #7678 remapped local debug file paths to build-dir-relative paths, which breaks IDE/GDB breakpoints in local builds.

Describe the solution (The How)

Apply -ffile-prefix-map=${REL_SRC_PATH}=. only when running on GitHub Actions (GITHUB_ACTIONS is set), while keeping local debug symbols mapped to real source paths.

Describe alternatives you've considered

• Remove -ffile-prefix-map entirely (keeps local debugging but drops CI/worktree ccache path normalization).

Testing

• cmake --preset linux-full -DLUA_DOCS_ON_BUILD=OFF
• cmake --build --preset linux-full --target cataclysm-bn-tiles cata_test-tiles
• ./out/build/linux-full/tests/cata_test-tiles "time_duration_to_string"
• Confirmed no ffile-prefix-map in out/build/linux-full/compile_commands.json in local env.

Additional context

LUA_DOCS_ON_BUILD was disabled for local verification because docs generation in this environment expects a root-level ./cataclysm-bn-tiles path and fails post-link.

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

Optional

• This is a PR that modifies build process or code organization.
• Please document the changes in the appropriate location in the docs/ folder.
• If documentation for this feature or process does not exist, please write it.
• If the change alters versions of software required to build or work with the game, please document it.

[scarf005]

on second thoughts, reverting this changes cause LLDB in local linux to not work