Fix sendmail executable permission check

[p-boenisch]

Fixes #7445

The previous permission check inspected the Unix mode bits directly and
could reject commands that were executable by the current process.

Use nix::unistd::access with AccessFlags::X_OK to check whether the
sendmail command is executable by the running process.

Checks performed:

• cargo fmt --check
• cargo check --features sqlite
• cargo test --features sqlite,mysql,postgresql
• cargo clippy --features sqlite,mysql,postgresql -- -D warnings

[stefan0xC]

As explained in the thread where I've suggested this fix #7445 (comment) this is not actually sufficient. Can you update your PR to use the access check from the suggested nix crate?

[p-boenisch]

Thanks for the correction. I've replaced the mode-bit check with nix::unistd::access using AccessFlags::X_OK and added nix as a Unix-only dependency with the fs feature. All tests and Clippy checks pass locally.