Running a command like `tailscale up --auth-key tskey-foo --auth-key tskey-bar` used to print
```
invalid value "tskey-bar" for flag -auth-key: flag provided multiple times
```
but now we print
```
invalid value "tskey-REDACTED" for flag -auth-key: flag provided multiple times
```
Fixes#18562
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>