CI: Always update the compiler cache

[LeonMatthesKDAB]

Previously, it would skip updating, as the key is always the same. That meant the cache got progressively older.

[LeonMatthesKDAB]

Before:

Run sccache --show-stats
Compile requests                   2308
Compile requests executed          1959
Cache hits                           47 <------------- YIKES!
Cache hits (C/C++)                   47
Cache misses                       1853
Cache misses (C/C++)               1438
Cache misses (Rust)                 415

After:

Run sccache --show-stats
Compile requests                   2308
Compile requests executed          1959
Cache hits                         1232 <-------------- MUCH BETTER!
Cache hits (C/C++)                  826
Cache hits (Rust)                   406
Cache misses                        668
Cache misses (C/C++)                659

:partying_face: