fix: skip cleanup if build is skipped

[mohammed90]

The env var XCADDY_SKIP_BUILD is supposed to imply XCADDY_SKIP_CLEANUP, but the implementation missed this detail. The Builder#Build method has defer buildEnv.Close(), which runs even on the early exit here:

xcaddy/builder.go

Lines 80 to 86 in c795956

	defer buildEnv.Close()
	if b.SkipBuild {
	log.Printf("[INFO] Skipping build as requested")
	return nil
	}

The environment#Close method does not know about (Builder).SkipBuild. We can overload the skipCleanup semantics because skipping build implies skipping cleanup.

[mholt]

Ah, thank you! Elegant fix. Didn't think about that defer.