[CI][C++] sccache not detected on all builds

[pitrou]

Describe the bug, including details regarding any error messages, version, and platform.

The conda-python Crossbow build detects and uses sccache:

-- Using sccache: /usr/local/bin/sccache

with actual cache hits:

=== sccache stats after the build ===
Compile requests                    482
Compile requests executed           482
Cache hits                          128
Cache hits (C/C++)                  128
Cache misses                        354
Cache misses (C/C++)                354
Cache hits rate                   26.56 %
Cache hits rate (C/C++)           26.56 %
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    3
Cache errors                          0
Compilations                        354
Compilation failures                  0
Non-cacheable compilations            0
Non-cacheable calls                   0
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.094 s
Average compiler                  6.539 s
Average cache read hit            0.044 s
Failed distributed compilations       0
Cache location                  s3, name: ***, prefix: /sccache/
Version (client)                0.12.0

but for some reason, the debian-python Crossbow build doesn't detect sccache and therefore falls backs on ccache with an empty cache:

-- Using ccache: /usr/bin/ccache

Yet, the sccache command seems to exist on the Docker image when run locally, and the GHA job defines the same environment variables (including SCCACHE_BUCKET).

Component(s)

Continuous Integration, C++

[pitrou]

Issue resolved by pull request 50992
#50992