Speed up CI builds with ccache (seeded from develop)

[marcocastignoli]

• This PR enables ccache for the main CMake build in CircleCI and stores the cache between runs.
• CircleCI now restores ~/.ccache on most branches from the latest cache produced on develop, so builds can reuse compiled objects.
• Runs on develop start from an empty cache and then save a fresh ~/.ccache snapshot (one per revision) to keep the seed cache updated.
• The build step also resets and prints ccache stats (ccache -z / ccache -s) so we can see cache hits in the logs.

For the purpose of testing instead of creating the cache from develop, the cache is created by this branch ci-ccache. We can later create another PR starting from this branch to see if the builds are indeed using the cache generated by ci-ccache, before merging this PR let's remember to fix the occurrences of ci-ccache in the code.

Results

In this PR, I edited a single file and the CI is successfully using ccache for all build job. These are the results of the speedup:

Job	ccache disabled	ccache enabled	speedup
b_ubu_force_release	10:52	01:02	11×
b_ubu_static_arm	20:55	01:09	18×
b_ubu_min_req	17:25	01:39	11×
b_ubu_ossfuzz	22:05	02:13	10×
b_ubu_min_req_clang	12:15	01:29	8×
b_ubu_clang	12:54	01:32	8×
b_win	23:06	05:47	4×
b_ubu	11:22	00:42	16×
b_ems	07:18	03:15	2×
b_osx	09:25	01:14	8×
b_ubu_static	11:16	00:42	16×
b_archlinux	18:49	01:29	13×

In total we save ~23 minutes (or around 2$) each run.

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[clonker]

won't that very easily break?

[marcocastignoli]

You mean the hard-coded path?

I cannot find a reliable way to find that path.

[clonker]

vswhere.exe might be able to do that for you (or vcvarsall or something)

[marcocastignoli]

This is the solution I found:

https://github.com/argotorg/solidity/pull/16385/changes#diff-269e9853066c3b757fe5118f39a0330b0fa38daa20092bb30657dbf79058f866R19-R25

[clonker]

some preliminary comments. very nice work altogether! i was thinking whether we should perhaps have another environment variable that globally enables or disables ccache entirely. that way, if there are problems, we can quickly switch it off.

[r0qs]

I'm wondering if this condition can be added to a command, maybe in ensure_ccache_key, and we could pass the list of branches to skip as parameters. In this way we can avoid some repetition in the config.

[marcocastignoli]

good idea! I did the same also for saving save_ccache_if_develop

[r0qs]

Why is this change necessary?

[marcocastignoli]

Because of CCACHE_BASEDIR,it needs to be an absolute path to keep ccache hash paths consistent across runs. it's probably not needed but it's cleaner imo

[r0qs]

Not directly related to this PR, but would it be better to define the versions as variables? That would make them easier to update in the future and help avoid potential mistakes when changing paths after a version bump.

[marcocastignoli]

Do we want to tackle this in this PR?