fix: use su with explicit exports for lifecycle hooks

[icholy]

Running commands with su discards the Env being set on the exec.Cmd. This results in no PATH being available so all commands must be fully qualified. The vscode devcontainers use docker's -u flag which doesn't have the same issue.

I ran into this incompatibility while trying to start a .devcontainer.json file generated by https://devenv.sh/

devenv init
vim devenv.nix # add devcontainer.enable = true
devenv shell # Ctrl-D to exit
devpod up . --debug

Generated .devcontainer.json:

{
  "customizations": {
    "vscode": {
      "extensions": [
        "mkhl.direnv"
      ]
    }
  },
  "image": "ghcr.io/cachix/devenv/devcontainer:latest",
  "overrideCommand": false,
  "updateContentCommand": "devenv test"
}

Error:

12:39:20 warn bash: line 1: devenv: command not found
12:39:20 debug Failed running postCreateCommand lifecycle script [su vscode -c devenv test]: exit status 127

Note:

The su command has a -p flag to preserve the environment. However, this flag omits the PATH, HOME, SHELL, and USER environment variables. So we'd need to use the export command for these either way.