ptyfwd: Always prioritize $SYSTEMD_TINT_BACKGROUND

By giving priority to --background= we prevent users from opting
out of coloring if an explicit color is chosen by a tool wrapping
one of our own tools. Instead, let's give priority to the environment
variable, so that even if our tools are wrapped by another tool with
a different background, users can still opt out of coloring just by
setting the environment variable, which has a high chance of being
forwarded to the invocation of our own tools which makes it easy to
use to disable color tinting globally if requested by the user.

docs/ENVIRONMENT.md

@@ -783,15 +783,14 @@ Tools using the Varlink protocol (such as `varlinkctl`) or sd-bus (such as
 
 `systemd-run`, `run0`, `systemd-nspawn`, `systemd-vmspawn`:
 
-* `$SYSTEMD_TINT_BACKGROUND` – Takes a boolean. When false the automatic
-  tinting of the background for containers, VMs, and interactive `systemd-run`
-  and `run0` invocations is turned off. Note that this environment variable has
-  no effect if the background color is explicitly selected via the relevant
-  `--background=` switch of the tool.
+* `$SYSTEMD_TINT_BACKGROUND` – Takes a boolean. When false the automatic and
+  explicit tinting of the background (via `--background=`) for containers, VMs,
+  `systemd-pty-forward` and interactive  `systemd-run` and `run0` invocations is
+  turned off.
 
 * `$SYSTEMD_ADJUST_TERMINAL_TITLE` – Takes a boolean. When false the terminal
-  window title will not be updated for interactive invocation of the mentioned
-  tools.
+  window title will not be updated for interactive invocation of the tools
+  mentioned above.
 
 `systemd-hostnamed`, `systemd-importd`, `systemd-localed`, `systemd-machined`,
 `systemd-portabled`, `systemd-timedated`: