fix(cmd/gno/clean): allow to run `gno clean -modcache` from anywhere + rename and use `gnomod.ModCachePath` + tmp `GNOHOME` in main tests #6005

Job	Run time
setup	5s
Run Main (gnodev) / Go Build / generated	12s
Run Main (gnodev) / Go Test / test	59s
Run Main (gnodev) / Go Linter / lint	35s
Run Main (gnofaucet) / Go Test / test	31s
Run Main (gnogenesis) / Go Build / generated	7s
Run Main (gnofaucet) / Go Build / generated	7s
Run Main (gnogenesis) / Go Linter / lint	22s
Run Main (gnofaucet) / Go Linter / lint	18s
Run Main (gnogenesis) / Go Test / test	31s
Run Main (gnokeykc) / Go Build / generated	10s
Run Main (gnokeykc) / Go Linter / lint	34s
Run Main (gnokeykc) / Go Test / test	35s
Run Main (gnomd) / Go Build / generated	11s
Run Main (gnomd) / Go Linter / lint	18s
Run Main (gnomd) / Go Test / test	28s
Run Main (gnomigrate) / Go Linter / lint	22s
Run Main (gnomigrate) / Go Test / test	28s
Run Main (gnomigrate) / Go Build / generated	11s
	7m 4s

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