pretty-print: show progress bar only when we are running on a TTY

pretty-print: show progress bar only when we are running on a TTY

Otherwise, when a command is running with e.g. StandardError=journal+console,
journal contains [xxxB blob data]:
```
[    5.628796] TEST-13-NSPAWN.sh[299]: + importctl import-raw --class=confext /var/tmp/importtest
[    5.632350] systemd-importd[302]: Successfully forked off '(sd-transfer)' as PID 319.
[    5.633671] TEST-13-NSPAWN.sh[318]: [83B blob data]
[    5.632598] (sd-transfer)[319]: Calling: /usr/lib/systemd/systemd-import raw --class confext - importtest
[    5.637769] systemd-importd[302]: (transfer1) Importing '/var/tmp/importtest', saving as 'importtest'.
[    5.637947] TEST-13-NSPAWN.sh[318]: [82B blob data]
[    5.638313] TEST-13-NSPAWN.sh[318]: [75B blob data]
[    5.638151] systemd-importd[302]: (transfer1) Operating on image directory '/var/lib/confexts'.
[    5.638863] systemd-importd[302]: (transfer1) Imported 40%.
[    5.638882] systemd-importd[302]: (transfer1) Wrote 40K.
[    5.639653] TEST-13-NSPAWN.sh[318]: [39B blob data]
[    5.639653] TEST-13-NSPAWN.sh[318]: [36B blob data]
[    5.639653] TEST-13-NSPAWN.sh[318]: [59B blob data]
[    5.639653] TEST-13-NSPAWN.sh[318]: [34B blob data]
[    5.638894] systemd-importd[302]: (transfer1) Operation completed successfully.
[    5.640760] TEST-13-NSPAWN.sh[318]: [25B blob data]
[    5.638902] systemd-importd[302]: (transfer1) Exiting.
```
The blob data entries are something like the following:
```
[    5.628796] TEST-13-NSPAWN.sh[299]: + importctl import-raw --class=confext /var/tmp/importtest
[    5.632350] systemd-importd[302]: Successfully forked off '(sd-transfer)' as PID 319.
[    5.633671] TEST-13-NSPAWN.sh[318]: ^M           ^MEnqueued transfer job 1. Press C-c to continue download in background.
[    5.632598] (sd-transfer)[319]: Calling: /usr/lib/systemd/systemd-import raw --class confext - importtest
[    5.637769] systemd-importd[302]: (transfer1) Importing '/var/tmp/importtest', saving as 'importtest'.
[    5.637947] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MImporting '/var/tmp/importtest', saving as 'importtest'.
[    5.638313] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MOperating on image directory '/var/lib/confexts'.
[    5.638151] systemd-importd[302]: (transfer1) Operating on image directory '/var/lib/confexts'.
[    5.638863] systemd-importd[302]: (transfer1) Imported 40%.
[    5.638882] systemd-importd[302]: (transfer1) Wrote 40K.
[    5.639653] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MImported 40%.
[    5.639653] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MWrote 40K.
[    5.639653] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MOperation completed successfully.
[    5.639653] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^M           ^MExiting.
[    5.638894] systemd-importd[302]: (transfer1) Operation completed successfully.
[    5.640760] TEST-13-NSPAWN.sh[318]: ^MTotal:   0%^MTotal:  40%
[    5.638902] systemd-importd[302]: (transfer1) Exiting.
```

Fixes #38552.