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/make.go

@@ -185,7 +185,7 @@ func (attr *MakeHelper) script(name string) string {
 
 	scriptMake := `
 make \
-	"-j$(nproc)"`
+	` + jobsFlagE
 	if len(attr.Make) > 0 {
 		scriptMake += " \\\n\t" + strings.Join(attr.Make, " \\\n\t")
 	}
@@ -193,7 +193,7 @@ make \
 
 	if !attr.SkipCheck {
 		scriptMake += attr.ScriptCheckEarly + `make \
-	"-j$(nproc)" \
+	` + jobsFlagE + ` \
 	`
 		if len(attr.Check) > 0 {
 			scriptMake += strings.Join(attr.Check, " \\\n\t")