create --*-from-command: run subcommands with a clean environment, fixes #7916

When borg invokes a system command, it needs to prepare the environment
for that. This is especially important when using a pyinstaller-made
borg fat binary that works with a modified env var LD_LIBRARY_PATH -
system commands may crash with that.

borg already had calls to prepare_subprocess_env at some places (e.g.
when invoking ssh for the remote repo connection), but they were
missing for:

borg create --content-from-command ...
borg create --paths-from-command ...

src/borg/archiver.py

@@ -533,7 +533,8 @@ class Archiver:
                 if not dry_run:
                     try:
                         try:
-                            proc = subprocess.Popen(args.paths, stdout=subprocess.PIPE, preexec_fn=ignore_sigint)
+                            env = prepare_subprocess_env(system=True)
+                            proc = subprocess.Popen(args.paths, stdout=subprocess.PIPE, env=env, preexec_fn=ignore_sigint)
                         except (FileNotFoundError, PermissionError) as e:
                             self.print_error('Failed to execute command: %s', e)
                             return self.exit_code
@@ -552,7 +553,8 @@ class Archiver:
                 paths_sep = eval_escapes(args.paths_delimiter) if args.paths_delimiter is not None else '\n'
                 if args.paths_from_command:
                     try:
-                        proc = subprocess.Popen(args.paths, stdout=subprocess.PIPE, preexec_fn=ignore_sigint)
+                        env = prepare_subprocess_env(system=True)
+                        proc = subprocess.Popen(args.paths, stdout=subprocess.PIPE, env=env, preexec_fn=ignore_sigint)
                     except (FileNotFoundError, PermissionError) as e:
                         self.print_error('Failed to execute command: %s', e)
                         return self.exit_code