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.

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+on: [push, pull_request]
+name: Continuous Integration
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  fw-ci:
+    name: Build
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./src/spice2x
+    steps:
+      - uses: actions/checkout@v4
+      - name: Compile
+        run: ./build_docker.sh
+      - uses: actions/upload-artifact@v4
+        with:
+          name: spice2x
+          path: src/spice2x/bin
+          if-no-files-found: error