internal/rosa: respect preferred job count

This discontinues use of nproc, and also overrides detection behaviour in ninja.

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

internal/rosa/rosa.go

@@ -602,6 +602,17 @@ cd '/usr/src/` + name + `/'
 	)
 }
 
+const (
+	// jobsE is expression for preferred job count set by [pkg].
+	jobsE = `"$` + pkg.EnvJobs + `"`
+	// jobsE is expression for flag with preferred job count.
+	jobsFlagE = `"-j$` + pkg.EnvJobs + `"`
+	// jobsLE is expression for twice of preferred job count set by [pkg].
+	jobsLE = `"$(expr ` + jobsE + ` '*' 2)"`
+	// jobsE is expression for flag with double of preferred job count.
+	jobsLFlagE = `"-j$(expr ` + jobsE + ` '*' 2)"`
+)
+
 // newTar wraps [pkg.NewHTTPGetTar] with a simpler function signature.
 func newTar(url, checksum string, compression uint32) pkg.Artifact {
 	return pkg.NewHTTPGetTar(nil, url, mustDecode(checksum), compression)