test: support sccache for pytest builds

[jpnurmi]

In the series of making it more pleasant to work on Windows, replace the previously undocumented ccache support with Windows-compatible sccache as the compilation cache for pytest builds, and document it in CONTRIBUTING.md.

sccache works on all platforms including Windows, making ccache redundant (added in #1529, was it used by anyone else?). On Windows, USE_SCCACHE=1 automatically switches to the Ninja generator (VS generator doesn't support compiler launchers) and uses /Z7 debug info format via CMP0141 policy (sccache can't handle /Zi shared PDBs).

YMMV, but on my old dedicated Windows laptop (Surface 4), this cuts the full pytest run from roughly 14-15 minutes to somewhere around 11-12 minutes:

pytest -v tests

(.venv) C:\Users\jpnurmi\Projects\sentry\sentry-native>pytest -v tests
=================================================================== test session starts ====================================================================
platform win32 -- Python 3.13.3, pytest-8.1.1, pluggy-1.6.0 -- C:\Users\jpnurmi\Projects\sentry\sentry-native\.venv\Scripts\python.exe
cachedir: .pytest_cache
rootdir: C:\Users\jpnurmi\Projects\sentry\sentry-native
plugins: flaky-3.8.1, pytest_httpserver-1.0.10, repeat-0.9.3, xdist-3.5.0
collected 1144 items
[...]
====================================================================== sccache stats =======================================================================
Compile requests                   3024
Compile requests executed          3024
Cache hits                         3023
Cache hits (C/C++)                 3023
Cache misses                          1
Cache misses (C/C++)                  1
Cache hits rate                   99.97 %
Cache hits rate (C/C++)           99.97 %
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Cache errors                          0
Compilations                          1
Compilation failures                  0
Non-cacheable compilations            0
Non-cacheable calls                   0
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.001 s
Average compiler                  0.866 s
Average cache read hit            0.009 s
Failed distributed compilations       0
Cache location                  Local disk: "C:\\Users\\jpnurmi\\AppData\\Local\\Mozilla\\sccache\\cache"
Base directories                (none)
Use direct/preprocessor mode?   yes
Version (client)                0.14.0
Cache size                           97 MiB
Max cache size                       10 GiB
===Flaky Test Report===

test_breakpad_dump_inflight passed 1 out of the required 1 times. Success!
test_shutdown_timeout passed 1 out of the required 1 times. Success!

===End Flaky Test Report===

======================================================= 1108 passed, 36 skipped in 653.14s (0:10:53) =======================================================

And in the CI (Note: these can vary a lot depending on the time of day):

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