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/llvm.go

@@ -190,7 +190,7 @@ ln -s ld.lld /work/system/bin/ld
 ln -s clang /work/system/bin/cc
 ln -s clang++ /work/system/bin/c++
 
-ninja check-all
+ninja ` + jobsFlagE + ` check-all
 `,
 	},
 		Python,
@@ -230,7 +230,7 @@ func (t Toolchain) newLibclc() (pkg.Artifact, string) {
 
 			{"LIBCLC_TARGETS_TO_BUILD", "all"},
 		},
-		Script: "ninja test",
+		Script: "ninja " + jobsFlagE + " test",
 	}), llvmVersion
 }
 func init() {