cmd/go/internal/modfetch: avoid path.Join in URL errors

path.Join collapses duplicated '/' tokens, but an HTTP URL
intentionally includes a '://' after the scheme.

This should fix the syntax of the errors seen in
https://build.golang.org/log/a17d0c7b6159ea4dad0a9e5378ab5a36ee30ce44.

Updates #52727.

Change-Id: I6e1773a7eb8b3e7e2b3ca29540c1e94a7cd2d99d
Reviewed-on: https://go-review.googlesource.com/c/go/+/461682
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Bryan C. Mills

Diff for: src/cmd/go/internal/modfetch/proxy.go

@@ -262,7 +262,7 @@ func (p *proxyRepo) getBytes(path string) ([]byte, error) {
 	if err != nil {
 		// net/http doesn't add context to Body errors, so add it here.
 		// (See https://go.dev/issue/52727.)
-		return b, &url.Error{Op: "read", URL: pathpkg.Join(p.redactedURL, path), Err: err}
+		return b, &url.Error{Op: "read", URL: strings.TrimSuffix(p.redactedURL, "/") + "/" + path, Err: err}
 	}
 	return b, nil
 }