CI - Runner caching with sccache

[bfops]

Description of Changes

This only changes Linux runners.

• use runner-cached sccache dir for most things
• use a runner-cached target directory for native/build-script artifacts
  • static CARGO_TARGET_DIR has been replaced with a step that sets it (because it is now based on HOME)

Timing results

These are sums of Cargo's reported Finished ... in ... times, matching the table in #5798. The test run was CI run 33413710732 on spacetimedb-new-runner-2-staging.

Job	Cargo time in master	this PR	Rust sccache hit rate
Test Suite	9m30s - 10m02s	6m44.9s	12.96%
Upload build artifacts (Linux)	2m27s - 2m46s	1m07s	93.33%
Lints	1m46s - 2m09s	55.6s	6.52%
Build smoketests (Linux)	48.9s - 57.1s	10.4s	83.33%
Check CLI docs	56s - 1m08s	13.0s	98.28%
Test spacetimedb-update flow (Linux)	29.6s - 30.7s	2.9s	100%
csharp-testsuite	28.2s - 46.6s	14.2s	0%
TypeScript - Tests	22.8s - 28.4s	2.3s	100%
Build and test wasm bindings	25.0s - 33.2s	3.3s	100%
Docs build	20s - 21s	1.2s	100%
Check that packages are publishable	19s - 21s	1.1s	not reported
godot-testsuite	7.6s - 11.2s	3.1s	100%

The next high-value change is to remove commit-specific metadata from shared Rust library artifacts (or otherwise confine it to a leaf artifact that does not participate in the test/lint dependency graph), then add the smoketest archive to the empty-target population phase.

API and ABI breaking changes

None.

Expected complexity level and risk

2

Testing

• the table is the testing

[jdetter]

This looks good to me, I didn't test anything manually here but the numbers in the table make sense and look fine to me. Thanks @bfops 👍