Clean up the build scripts

[neilenns]

Fixes #235

Description of changes

• Explicitly set the python version to use
• Add proper caching for PlatformIO

Build time without cache: 7 minutes 13 seconds
Build time with cache: 3 minutes 34 seconds

[github-actions]

Firmware for this pull request:
Firmware.zip

[neilenns]

First run of this PR shows no cache was present:

Cache not found for input keys: Linux-pio

Second run of this PR shows the cache being used:

Received 0 of 1725[13](https://github.com/MobiFlight/MobiFlight-FirmwareSource/actions/runs/6878073505/job/18706927963#step:4:14)376 (0.0%), 0.0 MBs/sec
Received 50331648 of 172513376 (29.2%), 23.8 MBs/sec
Received [14](https://github.com/MobiFlight/MobiFlight-FirmwareSource/actions/runs/6878073505/job/18706927963#step:4:15)3[15](https://github.com/MobiFlight/MobiFlight-FirmwareSource/actions/runs/6878073505/job/18706927963#step:4:16)3248 of 172513376 (83.0%), 45.2 MBs/sec
Cache Size: ~[16](https://github.com/MobiFlight/MobiFlight-FirmwareSource/actions/runs/6878073505/job/18706927963#step:4:17)5 MB (172513376 B)
/usr/bin/tar -xf /home/runner/work/_temp/81ee234b-19b0-4657-b240-b4b7daa7f1c5/cache.tzst -P -C /home/runner/work/MobiFlight-FirmwareSource/MobiFlight-FirmwareSource --use-compress-program unzstd
Cache restored successfully
Cache restored from key: Linux-pio

[github-actions]

Firmware for this pull request:
Firmware.zip

[neilenns]

Even with the cache being used I still see the pico-sdk getting cloned. I assume that is because it uses a different mechanism than other dependencies (cloned from a github repo instead of referenced from the platformio library). pico-sdk cloning is what is driving our long build times :(

[neilenns]

Found a thread about caching toolchains: https://community.platformio.org/t/caching-platformio-external-in-libs-gitlab-ci/12160/2, trying that.

[github-actions]

Firmware for this pull request:
Firmware.zip

[github-actions]

Firmware for this pull request:
Firmware.zip

[neilenns]

See https://github.com/MobiFlight/MobiFlight-FirmwareSource/actions/runs/6878419283/job/18708061068 for a build that used the cache and didn't re-clone the toolchains or frameworks.

[github-actions]

Firmware for this pull request:
Firmware.zip

[DocMoebiuz]

Thanks!