## Link to GitHub Issue or related Pull Request, if one exists
#0
## Description of change
`mingw-w64-libjpeg-turbo` and `mingw-w64-x264` are AUR packages, so the
deps image compiles them from source, once per mingw target arch. Stock
`makepkg.conf` leaves `MAKEFLAGS` commented out and both PKGBUILDs call
bare `make`, so all of that was building single-threaded.
Adds a `~/.makepkg.conf` for the build user setting
`MAKEFLAGS="-j$(nproc)"`. It is kept literal so it evaluates when
makepkg sources the file, rather than baking in the core count of
whichever machine built the image. Using `~/.makepkg.conf` rather than
`/etc` leaves the pacman-owned system file untouched.
Measured on 20 cores, compiling both libraries from scratch: **310s to
64s, a 4.8x improvement.**
Also merges the two `yay` invocations into a single layer.
## Testing
bicarusandGitHub
4c2a70bab6