cmd/dist: add "devel" substring check to isRelease computation

Non-release versions that are built from source without a VERSION file
specifying any particular version end up with a development version like
"devel go1.25-67e0681aef Thu Apr 24 12:17:27 2025 -0700". Right now
those versions are correctly determined to be non-release because they
don't have a "go" prefix, instead they have a "devel " prefix.

In preparation of being able to move the "devel" substring, add a check
that said substring isn't present anywhere, since it is certain not to
be included in any released Go version we publish at https://go.dev/dl/.

For #73372.

Change-Id: Ia3e0d03b5723d4034d6270c3a2224f8dfae380e9
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/667955
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: dmitshur

src/cmd/dist/build.go

@@ -274,7 +274,8 @@ func xinit() {
 	tooldir = pathf("%s/pkg/tool/%s_%s", goroot, gohostos, gohostarch)
 
 	goversion := findgoversion()
-	isRelease = strings.HasPrefix(goversion, "release.") || strings.HasPrefix(goversion, "go")
+	isRelease = (strings.HasPrefix(goversion, "release.") || strings.HasPrefix(goversion, "go")) &&
+		!strings.Contains(goversion, "devel")
 }
 
 // compilerEnv returns a map from "goos/goarch" to the