[CI] cache /nix/store with collect garbage

2 jobs for dev-spago-next-with-flake in 2 minutes and 55 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Compile
failed #12916
compile

00:02:55

 
  Test
skipped #12917
test
 
Name Stage Failure
failed
compile Compile
    at Module._load (node:internal/modules/cjs/loader:1022:12)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:135:12)
at node:internal/main/run_main_module:28:49

Node.js v20.12.2

error: install script from "cpu-features" exited with 1
Cleaning up file based variables
ERROR: Job failed: exit code 1