cmd/dist: permit go* tag in main branch when it includes "beta"

This change allows us to give an hg tag such as "go1.3beta1" to
revisions in the main branch without breaking the build.

This is helpful for community members who want to build the beta
from source.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/90190044

src/cmd/dist/build.c

@@ -444,7 +444,7 @@ setup(void)
 	}
 
 	// For release, make sure excluded things are excluded.
-	if(hasprefix(goversion, "release.") || hasprefix(goversion, "go")) {
+	if(hasprefix(goversion, "release.") || (hasprefix(goversion, "go") && !contains(goversion, "beta"))) {
 		for(i=0; i<nelem(unreleased); i++)
 			if(isdir(bpathf(&b, "%s/%s", goroot, unreleased[i])))
 				fatal("%s should not exist in release build", bstr(&b));