build: fail nicely if somebody runs all.bash from a binary tarball package
build: fail nicely if somebody runs all.bash from a binary tarball package

Fixes golang/go#20008.

Change-Id: I7a429490320595fc558a8c5e260ec41bc3a788e2
Reviewed-on: https://go-review.googlesource.com/41858
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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