add PostBuildCommand

[romanstingler]

I have implemented the post-build command feature. Here's what I did:

1. Added post-build command parsing in command_line.rs:

   • Added handling for the --post-build-command argument
   • The argument requires a value which will be stored in config.post_build_command

2. Created a post_build_command function in install.rs:

   • Similar to pre_build_command, it executes a shell command after successful package build
   • The command has access to PKGBASE and VERSION environment variables
   • The command runs in the package build directory

3. Added post-build command execution in install.rs:

   • Called post_build_command after a successful package build
   • Only executes if a post-build command is configured

4. Added post-build command config in config.rs:

   • Added PostBuildCommand option to the config file parser
   • The command can be specified in the config file under the [bin] section

Users can specify a post-build command in these ways:

1. Via command line: paru --post-build-command "your-command"
2. OR Via config file:

[bin]
PostBuildCommand = your-command

The command will receive:

• Current working directory set to the package build directory
• PKGBASE: The base package name
• VERSION: The package version

---

Tested with
cargo run -- -S vim-git --post-build-command "/tmp/code/paru/test.sh"
and
paru.conf

#
# Binary OPTIONS
#
[bin]
PostBuildCommand = "/tmp/code/paru/test.sh"

closes #1300

Waiting for feedback where to place
post_build_command(config, dir, base.package_base(), &version)?;