build(mac): drop webpack heap from 6 GB to 4 GB on the macOS build (#9968)

The 6 GB ceiling set by #9967 was too aggressive for the macOS x64
VM's total RAM. Build #1295 on `pgabf-macos-x64` failed in
`_build_runtime` at `unzip electron-vX.X.X-darwin-x64.zip` with exit
code 2 — never even reached webpack. That points at OS-level memory
pressure spilling out of the Node process and starving the rest of
the build: at 6 GB reserved, the box runs out of RAM long before
Terser actually needs the full ceiling.

Drop back to 4 GB, which still gives Terser a full extra gigabyte
beyond the original 3 GB setting that OOM-killed webpack in #1294,
but leaves enough headroom for the other steps in the appbundle
build to coexist.

Only the macOS appbundle path changes (see pkg/mac/build-functions.sh);
linux/pip/Makefile and dev-machine builds keep the 3 GB the `bundle`
npm script ships with.