linux: Fix "Failed to start language server" errors when starting Zed from .desktop file (#22335)

Closes #21406

Context:

A few weeks ago on Linux, we resolved an
[issue](https://github.com/zed-industries/zed/issues/20070) where users
could not open more than one file from the file explorer. This was fixed
by replacing `zed-editor` (zed binary in the code) with `zed` (cli
binary in the code) in the `.desktop` file. The reason for this change
was that using the cli to open files is more convenient - it determines
weather to spawn a new Zed instance or use an existing one, if we use
main binary instead it would throw error `Zed is already running`.

You can read the complete PR here: [linux: Fix file not opening from
file explorer](https://github.com/zed-industries/zed/pull/21137).

While this fix resolved the original issue, it introduced a new one.

Problem:

When the cli binary is used, it assumes it is always being invoked from
a terminal and relies on `std::env::vars()` to retrieve the environment
variables needed to spawn Zed. These env vars are then passed to the
worktree, and eventually, languages use the `PATH` from this env to find
binaries. This leads to the "Failed to start language server" error when
the `.desktop` entry is used on Linux.

Solution:

When the `zed-editor` binary is used, it uses some clever Unix-specific
logic to retrieve the default shell (`load_shell_from_passwd`) and then
fetch the env vars from that shell (`load_login_shell_environment`).
This same logic should be used in the cli binary when it is invoked via
a `.desktop` entry rather than from a terminal.

Approach:

I moved these two functions mentioned above to a utils file and reused
them in cli binary to fetch env vars only on Linux when it is not run
from a terminal. This provides missing paths, and fix the issue.

It is also possible to handle this in the `zed-editor` binary by
modifying the logic in `handle_cli_connection`, where `CliRequest::Open`
is processed. There we can discard incoming env, and use our logic. But
discarding incoming envs felt weird, and I thought it's better to handle
this at source.

Release Notes:

- Fixed `Failed to start language server` errors when starting from
dekstop entry on Linux

crates/zed/Cargo.toml

@@ -68,7 +68,6 @@ language_models.workspace = true
 language_selector.workspace = true
 language_tools.workspace = true
 languages = { workspace = true, features = ["load-grammars"] }
-libc.workspace = true
 log.workspace = true
 markdown.workspace = true
 markdown_preview.workspace = true