fix(cmd/gno/clean): allow to run `gno clean -modcache` from anywhere + rename and use `gnomod.ModCachePath` + tmp `GNOHOME` in main tests #8983

Job	Run time
Run Main (autocounterd) / Go Build / generated	7s
Run Main (autocounterd) / Go Linter / lint	57s
Run Main (autocounterd) / Go Test / test	1m 4s
Run Main (docker-integration) / Go Build / generated	18s
Run Main (docker-integration) / Go Linter / lint	8s
Run Main (docker-integration) / Go Test / test	13s
Run Main (genproto) / Go Build / generated	15s
Run Main (genproto) / Go Linter / lint	29s
Run Main (genproto) / Go Test / test	31s
Run Main (genstd) / Go Build / generated	10s
Run Main (genstd) / Go Linter / lint	29s
Run Main (genstd) / Go Test / test	44s
Run Main (goscan) / Go Build / generated	6s
Run Main (goscan) / Go Linter / lint	12s
Run Main (goscan) / Go Test / test	13s
Run Main (loop) / Go Build / generated	10s
Run Main (loop) / Go Linter / lint	32s
Run Main (loop) / Go Test / test	46s
	7m 24s

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