cmd/go: don't fail on missing runtime/internal/sys/zversion.go
cmd/go: don't fail on missing runtime/internal/sys/zversion.go

The generated file runtime/internal/sys/zversion.go is deleted by
`go tool cmd dist clean` as part of running clean.bash. Don't treat
a missing file as a reason to stop running the go tool; just treat
is as meaning that runtime/internal/sys is stale.

No test because I don't particularly want to clobber $GOROOT.

Fixes #20385.

Change-Id: I5251a99542cc93c33f627f133d7118df56e18af1
Reviewed-on: https://go-review.googlesource.com/43559
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

vsekhar/elastic-go,golang/go,derekmarcotte/go,akutz/go,codestation/go,CAFxX/go,josharian/go,nawawi/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,codestation/go,ImJasonH/go,polyverse-security/go,ImJasonH/go,nawawi/go,codestation/go,akutz/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,CAFxX/go,Cofyc/go,polyverse-security/go,Cofyc/go,josharian/go,josharian/go,codestation/go,CAFxX/go,ImJasonH/go,akutz/go,Cofyc/go,codestation/go,polyverse-security/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,josharian/go,polyverse-security/go,akutz/go,nawawi/go,ImJasonH/go,josharian/go,derekmarcotte/go,derekmarcotte/go,ImJasonH/go,codestation/go,nawawi/go,codestation/go,golang/go,josharian/go,CAFxX/go,nawawi/go,josharian/go,christopher-henderson/Go,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,CAFxX/go,Cofyc/go,josharian/go,ImJasonH/go,vsekhar/elastic-go,akutz/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,golang/go,polyverse-security/go,akutz/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,codestation/go,polyverse-security/go,golang/go,derekmarcotte/go,vsekhar/elastic-go,cloudflare/go,golang/go,golang/go,derekmarcotte/go,akutz/go,Cofyc/go,golang/go,cloudflare/go,cloudflare/go,derekmarcotte/go,Cofyc/go,nawawi/go,ImJasonH/go,akutz/go,derekmarcotte/go