CI: replace cachepot with sccache

[bayandin]

Problem

Cachepot development has been suspended, from their README:

If you're still using cachepot, at this point, all of the relevant changes should be backported to sccache or are in progress. Feel free to submit a porting request at sccache referencing a PR in cachepot.

Results with sccache:

job	no cache1	sccache2
build-neon (debug)*	2m	2m 4s
build-neon (release)*	3m 12s	1m 42s
neon-arch-image (x64)	7m 6s	3m 55s
neon-arch-image (arm64)	13m 8s	5m 59s
compute-node-image-arch (v14, x64)	4m 41s	2m 39s
compute-node-image-arch (v14, arm64)	10m 2s	6m 8s
compute-node-image-arch (v15, x64)	4m 45s	2m 35s
compute-node-image-arch (v15, arm64)	9m 34s	5m 53s
compute-node-image-arch (v16, x64)	4m 48s	2m 48s
compute-node-image-arch (v16, arm64)	10m 2s	6m 9s

• cargo build-like steps only

1. no cache: https://github.com/neondatabase/neon/actions/runs/9717370650/job/26828975170
2. sccache: https://github.com/neondatabase/neon/actions/runs/9732037662/job/26857211223

Summary of changes

• Replace cachepot with sccache
• Set correct env vars to make sccache work

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2940 tests run: 2823 passed, 0 failed, 117 skipped (full report)

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Code coverage* (full report)

• functions: 32.7% (6902 of 21123 functions)
• lines: 50.1% (54129 of 108144 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6fca885 at 2024-06-30T13:25:46.358Z :recycle:}