feat: purge build cache

[Bikappa]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

The build files saved under temporary folders gets cleaned regularly if not used.

What is the current behavior?

Unused files remain stale in temporary folders until the os decides to eventually remove them

What is the new behavior?

After an amount of compilations the built files are deleted if they were last used before a period of time.
The number compilations before the purge is triggered and the maximum age of the files are configurable with the following environment variables:

• ARDUINO_BUILD_CACHE_COMPILATIONS_BEFORE_PURGE (0 signifies never purge, default to 10)
• ARDUINO_BUILD_CACHE_TTL (default to 30 days)

This does only apply the default, temporary, build path. It does not purge files under user-specified locations, outside of the default one.

The default directory structures is also changed from:

arduino
├── core-cache
│   ├── core-x.a
│   ├── core-y.a
│   └── core-z.a
├── sketch-1
├── sketch-2
└── sketch-3

to

arduino
├── cores
│   ├── x
│   │   └── core.a
│   ├── y
│   │   └── core.a
│   └── z
│       └── core.a
└── sketches
    ├── 1
    ├── 2
    └── 3

Does this PR introduce a breaking change, and is titled accordingly?

Other information

Fixes #2029

[codecov]

Codecov Report

Base: 36.68% // Head: 36.74% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (4c64cb5) compared to base (396718f).
Patch coverage: 37.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2033      +/-   ##
==========================================
+ Coverage   36.68%   36.74%   +0.06%     
==========================================
  Files         228      229       +1     
  Lines       19385    19456      +71     
==========================================
+ Hits         7111     7150      +39     
- Misses      11442    11472      +30     
- Partials      832      834       +2     

Flag	Coverage Δ
unit	36.74% <37.50%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commands/compile/compile.go	0.00% <0.00%> (ø)
buildcache/build_cache.go	46.34% <46.34%> (ø)
legacy/builder/phases/core_builder.go	58.25% <58.33%> (-0.52%)	⬇️
arduino/sketch/sketch.go	63.79% <100.00%> (ø)
configuration/defaults.go	100.00% <100.00%> (ø)
...egacy/builder/add_additional_entries_to_context.go	62.50% <100.00%> (ø)
commands/lib/search.go	92.10% <0.00%> (+3.94%)	⬆️
arduino/monitor/monitor.go	47.36% <0.00%> (+6.31%)	⬆️

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[cmaglie]

The go-paths can be exploited much more here, I didn't comment everywhere, but you surely got the pattern :-)

[per1234]

UPDATE: resolved by 7082f17

Please add the new `build_cache.compilations_before_purge` and `build_cache.ttl` configuration keys to the documentation:

https://github.com/arduino/arduino-cli/blob/feat/purge-build-cache/docs/configuration.md

[per1234]

The .last-used file is not being written to the core cache. This causes the core to always be considered older than TTL.

[Bikappa]

@cmaglie @per1234 I went through your comments 👍 and updated accordingly

[per1234]

This is a very nice system. Thanks Luca!