chore: Do not bust Rust build cache when opening projects with dev build (#26278)

## Problem
Running `cargo run .` twice in Zed repository required a rebuild two
times in a row. The second rebuild was triggered around libz-sys, which
in practice caused a rebuild of the ~entire project.

Some concrete examples:
```
cargo test -p project # Requires a rebuild (warranted)
cargo run .
cargo test -p project # Requires a rebuild (unwarranted)
```
or
```
cargo run . # Requires a rebuild (warranted)
cargo run . # Requires a rebuild (unwarranted)
```

## What's going on
Zed build script on MacOS sets MACOSX_DEPLOYMENT_TARGET to 10.15. This
is fine. However, **cargo propagates all environment variables to child
processes during `cargo run`**. This then affects Rust Analyzer spawned
by dev Zed - it clobbers build cache of whatever package it touches,
because it's behavior is not same between running it with `cargo run`
(where MACOS_DEPLOYMENT_TARGET gets propagated to child Zed) and running
it directly via `target/debug/zed` or whatever (where the env variable
is not set, so that build behaves roughly like Zed Dev.app).


## Solution
~We'll unset that env variable from user environment when we're
reasonably confident that we're running under `cargo run` by exploiting
other env variables set by cargo:
https://doc.rust-lang.org/cargo/reference/environment-variables.html
CARGO_PKG_NAME is always set to `zed` when running it via `cargo run`,
as it's the value propagated from the build.~

~The alternative I've considered is running [via a custom
runner](https://doc.rust-lang.org/cargo/reference/config.html#targetcfgrunner),
though the problem here is that we'd have to use a shell script to unset
the env variable - that could be problematic with e.g. fish. I just
didn't want to deal with that, though admittedly it would've been
cleaner in other aspects.~

Redact all above. We'll just set MACOSX_DEPLOYMENT_TARGET regardless of
whether you have it in your OG shell environment or not.

Release Notes:

- N/A

.cargo/config.toml

@@ -26,3 +26,6 @@ rustflags = [
     "-C",
     "target-feature=+crt-static", # This fixes the linking issue when compiling livekit on Windows
 ]
+
+[env]
+MACOSX_DEPLOYMENT_TARGET = "10.15.7"