Fix issue when changing user in exec subcommands

[sabre1041]

Fix exec subcommands (exec-file and exec-env) to set HOME, USER, and LOGNAME environment variables in the child process when the --user flag is used.

Originating Issue

When using --user to run a child process as a different user, SwitchUser() changes the UID/GID but leaves the environment variables pointing at the original user. This causes the child process to inherit incorrect values for HOME,
USER, and LOGNAME, which can break tools that rely on these variables (e.g., resolving ~ or determining the current user).

Example of issue:

sops exec-env --user=sops test.yml bash
bash: /root/.bashrc: Permission denied

Changes

• Added UserEnv() function in exec_unix.go that looks up the target user and returns the correct HOME, USER, and LOGNAME environment variables
• Updated ExecWithFile() and ExecWithEnv() to capture user environment variables before switching the user, then inject them into the child process environment
• User environment variables are included in both pristine and non-pristine modes when --user is set
• Added a stub UserEnv() on Windows (exec_windows.go) that fatals, consistent with the existing SwitchUser() behavior
• Added unit tests in exec_unix_test.go covering UserEnv() output, process environment isolation, and integration with both exec modes in pristine and non-pristine configurations

[felixfontein]

Thanks for working on this!

[felixfontein]

Assuming you did run the root tests (I don't think they run in CI), this looks good!