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/ninja.go

@@ -15,7 +15,7 @@ cd "$(mktemp -d)"
 python3 /usr/src/ninja/configure.py \
 	--bootstrap \
 	--gtest-source-dir=/usr/src/googletest
-./ninja all
+./ninja `+jobsFlagE+` all
 ./ninja_test
 
 mkdir -p /work/system/bin/