🌌 Use build.rustflags (and variants) as a replacement for environment variable to enable spaces in paths

[EmilioLaiso]

Fixes rust-mobile#128

As per the docs https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags

There are four mutually exclusive sources of extra flags. They are checked in order, with the first one being used:

- CARGO_ENCODED_RUSTFLAGS environment variable.
- RUSTFLAGS environment variable.
- All matching target.<triple>.rustflags and target.<cfg>.rustflags config entries joined together.
- build.rustflags config value.

Using the build.rutflag approach allows for spaces in paths, as seen in https://github.com/rust-lang/docs.rs/blob/bcc1a2c9f63a62907abbe68dcbe3f73632a9a4de/crates/metadata/lib.rs#L256-L262

Additionally, as suggested in rust-mobile#128 (comment),
setting CC_SHELL_ESCAPED_FLAGS=1 and inserting quotes around paths that are passed to C(XX)FLAGS solves all the remaining issues (provided that cc-rs is updated to 1.1.11)

[EmilioLaiso]

Should I link

rust-lang/cargo#14398
and
rust-mobile#186

?

[MarijnS95]

Should I link

rust-lang/cargo#14398
and
rust-mobile#186

?

Only if it's merged to xbuild main? Might want to reopen the PR there too for the larger community.