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/kernel.go

@@ -1221,7 +1221,7 @@ install -Dm0500 \
 	/sbin/depmod
 
 make \
-	"-j$(nproc)" \
+	` + jobsFlagE + ` \
 	-f /usr/src/kernel/Makefile \
 	O=/tmp/kbuild \
 	LLVM=1 \
@@ -1309,7 +1309,7 @@ func (t Toolchain) newFirmware() (pkg.Artifact, string) {
 			"install-zst",
 		},
 		SkipCheck: true, // requires pre-commit
-		Install:   `make "-j$(nproc)" DESTDIR=/work/system dedup`,
+		Install:   "make " + jobsFlagE + " DESTDIR=/work/system dedup",
 	},
 		Parallel,
 		Rdfind,