Add CACHEDIR.TAG to .embuild/espressif directories

[drinkcat]

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Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
• [*] I have used cargo fmt command to ensure that all changed code is formatted correctly.
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Pull Request Details 📖

Description

On Linux, this is the standard way to prevent cache directories from being included in backups, and mirrors what cargo does (look at target directory).

cargo does quite a bit more though, including preventing backup on MacOS and indexing on Windows, we might want to pull in more of the code (sadly we can't directly pull these functions from cargo-util).

I only did this for espidf and pio, not sure if it makes sense elsewhere.

Testing

In a personal project, add to Cargo.toml:

[patch.crates-io]
embuild = { path = "../embuild" }

Then cargo build:

$ cat .embuild/espressif/CACHEDIR.TAG 
Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by embuild.
# For information about cache directory tags see https://bford.info/cachedir/