Extract correct version when using --with flag to replace Caddy core

[jshenguru]

The following build failed on Windows with Invalid Semantic Version error.
xcaddy build v2.8.4 --with github.com/caddyserver/caddy/v2=c:\Users\test\caddy

The bug is among lines 97 - 110 in the builder.go.

	// generating windows resources for embedding
	if b.OS == "windows" {
		// get version string, we need to parse the output to get the exact version instead tag, branch or commit
		cmd := buildEnv.newGoBuildCommand(ctx, "list", "-m", buildEnv.caddyModulePath)
		var buffer bytes.Buffer
		cmd.Stdout = &buffer
		err = buildEnv.runCommand(ctx, cmd)
		if err != nil {
			return err
		}

		// output looks like: github.com/caddyserver/caddy/v2 v2.7.6
		version := strings.TrimSpace(strings.TrimPrefix(buffer.String(), buildEnv.caddyModulePath))

		err = utils.WindowsResource(version, outputFile, buildEnv.tempFolder)

Detailed explanation:
The go list -m command in this case would return github.com/caddyserver/caddy/v2 v2.8.4 => c:\Users\test\caddy. The value of the version would end up being v2.8.4 => c:\Users\test\Documents\EyeSCADA-v4\_esRouter\caddy-2.8.4. Therefore, the utils.WIndows Resource() call would fail with 'Invalid Semantic Version` error.

A quick fix would be adding the following between version := ... and err = util.WIndowsResource(...):

index := strings.Index(version, "=>")
if index != -1 {
    version = strings.TrimSpace(version[:index])
}

[mholt]

Cool, thanks for the patch. LGTM