ci: name the build tool in one place, and remove a pin that resolved to 2026-05

[Sunrisepeak]

Every pull request opened after 2026-07-11 fails at mcpp build with

[error] package 'xim:glibc@>=2.39' not found
[error] package 'xim:gcc-runtime@>=15' not found
error: auto-installing default toolchain gcc@16.1.0 failed

before a line is compiled, so the failure belongs to no change under review. #12 is red for this reason alone.

Cause

.xlings.json declared workspace.mcpp = 0.0.13. That file is read from the current directory, and actions/checkout places it there before any step runs, so every mcpp invocation in this repository resolved to a build tool from 2026-05 rather than to the one the job had installed. The index no longer serves the toolchain packages a tool of that age asks for.

Reproduced outside CI. In a clean worktree of master:

$ mcpp --version
[warn] mcpp@0.0.13 is the version this project asks for, and it is not installed yet
         from                  ./.xlings.json  ->  workspace.mcpp
$ rm .xlings.json && mcpp --version
mcpp 2026.8.28.2

What changes

• The pin file is removed and the version becomes env.MCPP_VERSION, read by both the install and the cache key. They named different versions before, which is why Cache not found for input keys: mcpp-sandbox-Linux-mcpp0.0.13 appears in every log: that cache had never once been hit.
• mcpp --version is printed. The version a job runs is now the version it reports.
• mcpp self config --mirror GLOBAL, which is what a runner outside China needs.
• A weekly schedule. The breakage occurred between 2026-07-11 and 2026-08-29 and nothing reported it, because master received no pushes in that window. The first run to meet it was a contributor's, who had no way to distinguish a broken environment from a broken change.
• compile_commands.json is ignored. mcpp build writes it beside the manifest.

Measured

Locally against mcpp 2026.8.28.2, in a clean worktree of master:

   Compiling tinyhttps v0.2.9 (.)
   Compiling compat.mbedtls v3.6.1
    Finished dev [unoptimized + debuginfo] in 2.46s

[==========] 9 tests from 3 test suites ran. (18958 ms total)
[  PASSED  ] 9 tests.
 test result ok. 2 passed; 0 failed

CI on this branch is the criterion for the pinned version, 2026.8.29.1, which is not the one measured above.