cmd/gorebuild: drop "@" prefix in defaultVersions

When arguments are provided to gorebuild, the "@" character can be used
to specify a version. Otherwise version selection happens automatically
via defaultVersions. Its output are Go versions, no need for any prefix.

Fixes the error preventing gorebuild from running when versions are not
explicitly provided via arguments:

	$ gorebuild
	18:05:05.812 downloaded https://go.dev/dl/?mode=json&include=all
	18:05:05.836 FAIL: unknown version "@go1.21.0"

For golang/go#57120.

Change-Id: I050bd9d6d12d89b6891c845e686326c87eae5716
Reviewed-on: https://go-review.googlesource.com/c/build/+/517515
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dmitshur

cmd/gorebuild/report.go

@@ -305,7 +305,7 @@ func defaultVersions(releases []*DLRelease) []string {
 				// not reproducible
 				break
 			}
-			versions = append(versions, "@"+r.Version)
+			versions = append(versions, r.Version)
 			seen[major] = true
 			if len(seen) == 2 {
 				break