handle --long build flags properly

cmd/go treats "--foo=bar" juts like "-foo=bar",
just like any other program using the flag package.

However, we didn't support this longer form in filterForwardBuildFlags.
Because of it, "garble build -tags=foo" worked,
but "garble build --tags=foo" did not,
as we wouldn't forward "--tags=foo" as a build flag for "go list".

Fixes #429.

main.go

@@ -1880,7 +1880,10 @@ func filterForwardBuildFlags(flags []string) (filtered []string, firstUnknown st
 		arg := flags[i]
 		name := arg
 		if i := strings.IndexByte(arg, '='); i > 0 {
-			name = arg[:i]
+			name = arg[:i] // "-name=value" to "-name"
+		}
+		if strings.HasPrefix(name, "--") {
+			name = name[1:] // "--name" to "-name"
 		}
 
 		buildFlag := forwardBuildFlags[name]

testdata/scripts/imports.txt

@@ -36,8 +36,9 @@ go build -tags buildtag
 exec ./main
 cmp stdout main.stdout
 
-# Also check that -literals doesn't break anything
-garble -literals build -tags buildtag
+# Check that -literals doesn't break anything.
+# Also check that a different form of -tags still works.
+garble -literals build --tags=buildtag
 exec ./main
 cmp stdout main.stdout