chore: let -j govern per-recipe build parallelism

.config pinned COOKBOOK_MAKE_JOBS=4. config.rs reads that variable from
the environment into cook_opt.jobs, and cook_build.rs exports it to every
recipe's build script as its -j value; mk/repo.mk passes no --jobs flag,
so the pin was authoritative. Every recipe therefore compiled with -j4 no
matter what -j build-redbear.sh was given, capping a 24-core host at ~12
concurrent compiles through Qt, KF6 and Mesa.

Unset, it derives from -j (default available_parallelism()). Re-add only
to deliberately clamp per-recipe compiles BELOW -j.

.config

@@ -1,3 +1,8 @@
 PODMAN_BUILD?=0
 REDBEAR_ALLOW_PROTECTED_FETCH=1
-COOKBOOK_MAKE_JOBS=4
+# COOKBOOK_MAKE_JOBS intentionally unset: the cookbook derives it from the
+# build-redbear.sh -j value (cook_build.rs sets it from cli_jobs), so the single
+# -j flag governs both recipe-level and per-recipe parallelism. Hardcoding it
+# here silently capped EVERY recipe's compile at that value no matter what -j
+# was passed, which on a many-core host left most of the machine idle.
+# Re-add only to deliberately clamp per-recipe compiles below -j.