Add CI and improve build system (#293)

## Link to GitHub Issue, if one exists
Fixes #292

## Description of change
This PR adds CI with Github Actions. Runs on every push and pull
request. If a new commit is pushed, any previous runs are cancelled for
that branch.

In total, these optimisations (on a 7800X3D with 32GB 6000MT/s RAM and
an NVMe Gen4 SSD):
* Reduces `spicetools/deps` image size by around 200 MB, and build time
by almost 50%
* Reduces compilation time by around 9%

Changes:
* Pull archlinux image from GHCR so that we don't run into pull limits
from docker.io
* Use `base-devel` archlinux image
* Reduces image build time from 141 seconds to 109 seconds on my machine
  * Clean up unneeded packages
* Small reduction in `spicetools/deps` image size (4.35 GB vs 4.14 GB)
* Use precompiled `yay` binary
* Reduces image build time from 163 seconds to 141 seconds on my machine
* Use `nproc` to get the number of cores instead of parsing
`/proc/cpuinfo` with `awk`
* Use `ninja` to compile
  * Speeds up compile times a little, around 13 seconds on my machine
* Don't run `docker run` in interactive TTY in `build_docker.sh`
  * Github Actions disallows interactive TTYs
  * Still allowed for the batch script
* `time` the compilation process
* Enable BuildKit in the bash script
  * Required for build contexts
* Don't use `USER` instruction
  * Breaks Github Actions, doesn't seem to affect build?
* More info:
https://docs.github.com/en/actions/sharing-automations/creating-actions/dockerfile-support-for-github-actions#user
  * `USER` also seems to cause issues on a mounted ReFS image
* Add executable bits to bash scripts

Future ideas:
* Combine `external/Dockerfile` and `./Dockerfile` into one to
deduplicate images and save disk space
* Fix the weird `/src/src` directory structure in the Dockerfile and
build scripts
* Investigate CMake and ninja optimisations?

## Compiling
Check CI status on my fork.

## Testing
Tested with docker on Windows. Also play tested a bit to make sure the
Github Actions artifact works.

src/spice2x/external/docker/Dockerfile

@@ -1,20 +1,11 @@
-FROM archlinux:base
+FROM ghcr.io/archlinux/archlinux:base-devel
 
-RUN pacman --noconfirm -Syu gettext \
-    base-devel \
-    git \
+RUN pacman --noconfirm -Syu git \
     bash \
     zip \
     upx \
-    sudo \
-    binutils \
-    file \
-    make \
-    gcc \
-    fakeroot \
-    diffutils \
+    ninja \
     cmake \
-    awk \
     unzip \
     mingw-w64-crt \
     mingw-w64-winpthreads \
@@ -22,4 +13,4 @@ RUN pacman --noconfirm -Syu gettext \
     mingw-w64-headers \
     mingw-w64-binutils
 RUN useradd user -m && echo "ALL ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
-RUN su user -c "cd /tmp/ && git clone https://aur.archlinux.org/yay.git --depth=1 && cd yay && makepkg --noconfirm -si && yay --noconfirm -S mingw-w64-cmake"
+RUN su user -c "cd /tmp/ && git clone https://aur.archlinux.org/yay-bin.git --depth=1 && cd yay-bin && makepkg --noconfirm -si && yay --noconfirm -S mingw-w64-cmake"