internal/fetch: fix bug setting environment

We were adding "GOROOT=" and the path as separate elements to the
environment slice rather than adding the single "GOROOT=<path>" element.

Fixes #70581

Change-Id: I9f8afa271c8bae38ac37d1476eef4f3a532798d6
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/635675
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>
kokoro-CI: kokoro <[email protected]>

internal/fetch/getters.go

@@ -287,7 +287,7 @@ func NewGoPackagesStdlibModuleGetter(ctx context.Context, dir string) (*goPackag
 	start := time.Now()
 	env := []string(nil)
 	if dir != "" {
-		env = append(os.Environ(), "GOROOT=", abs)
+		env = append(os.Environ(), "GOROOT="+abs)
 	}
 	cfg := &packages.Config{
 		Context: ctx,