system: Define default 'PS1' in /etc/bashrc rather than ~/.bashrc.

Users can override 'PS1' in ~/.bashrc if they wish.

Previously, on Guix Home, the "default" 'PS1' would be set in ~/.bashrc
when 'home-bash-configuration-guix-defaults?' is true, preventing users
from overriding it via the 'environment-variables' field of
'home-bash-extension'.

* gnu/system/shadow.scm (%default-bashrc): Remove 'PS1' setting.
* gnu/system.scm (operating-system-etc-service): Define PS1 in
/etc/bashrc.
* gnu/home/services/shells.scm (add-bash-configuration): When
'home-bash-configuration-guix-defaults?' is true, add a default 'PS1' to
~/.bash_profile.

gnu/home/services/shells.scm

@@ -407,6 +407,15 @@ if [ -f ~/.profile ]; then source ~/.profile; fi
 # Honor per-interactive-shell startup file
 if [ -f ~/.bashrc ]; then source ~/.bashrc; fi
 "
+
+        ;; The host distro might provide a bad 'PS1' default--e.g., not taking
+        ;; $GUIX_ENVIRONMENT into account.  Provide a good default here when
+        ;; asked to.  The default can be overridden below via
+        ;; 'environment-variables'.
+        (if (home-bash-configuration-guix-defaults? config)
+            "PS1='\\u@\\h \\w${GUIX_ENVIRONMENT:+ [env]}\\$ '\n"
+            "")
+
         (serialize-field 'bash-profile)
         (serialize-field 'environment-variables)))
 

gnu/system.scm

@@ -1047,6 +1047,9 @@ fi
         (bashrc    (plain-file "bashrc" "\
 # Bash-specific initialization.
 
+# Provide a default prompt.  The user's ~/.bashrc can override it.
+PS1='\\u@\\h \\w${GUIX_ENVIRONMENT:+ [env]}\\$ '
+
 # The 'bash-completion' package.
 if [ -f /run/current-system/profile/etc/profile.d/bash_completion.sh ]
 then

gnu/system/shadow.scm

@@ -142,13 +142,6 @@ fi
 # Source the system-wide file.
 [ -f /etc/bashrc ] && source /etc/bashrc
 
-# Adjust the prompt depending on whether we're in 'guix environment'.
-if [ -n \"$GUIX_ENVIRONMENT\" ]
-then
-    PS1='\\u@\\h \\w [env]\\$ '
-else
-    PS1='\\u@\\h \\w\\$ '
-fi
 alias ls='ls -p --color=auto'
 alias ll='ls -l'
 alias grep='grep --color=auto'\n"))