cmd/go: fix cgo prefix mapping for non-vendored modules

Regression introduced in CL 478455.

Change-Id: I8083a0accb047188dde1a3d408df2aafaf4a5356
Reviewed-on: https://go-review.googlesource.com/c/go/+/493835
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>

src/cmd/go/internal/work/exec.go

@@ -2612,7 +2612,7 @@ func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []s
 				from = m.Dir
 				toPath = m.Path
 				if m.Version != "" {
-					m.Path += "@" + m.Version
+					toPath += "@" + m.Version
 				}
 			}
 			// -fdebug-prefix-map (or -ffile-prefix-map) requires an absolute "to"