internal/rosa: expose in-place behaviour in generic helper

This change also combines the createDir and wantsDir methods, and replaces the non-inplace target of the generic helper with a deterministic path.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/package/nss.az

@@ -78,7 +78,9 @@ package nss-cacert {
 
 	source = nss;
 
+	enterSource = true;
 	exec = generic {
+		inPlace = true;
 		build = `
 mkdir -p /work/system/etc/ssl/{certs/unbundled,certs/hashed,trust-source}
 buildcatrust \