shell.nix: change prompt color to magenta

"Light gray" underperforms in light themes.

Change-Id: I03296f04593975cf912da83f3cc5192c7ccb048a
Reviewed-on: https://review.monogon.dev/c/monogon/+/2081
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/shell.nix b/shell.nix
index d64223f..b0d4e97 100644
--- a/shell.nix
+++ b/shell.nix
@@ -10,7 +10,7 @@
     # Fancy colorful PS1 to make people notice easily they're in the Monogon Nix shell.
     PS1='\[\033]0;\u/monogon:\w\007\]'
     if type -P dircolors >/dev/null ; then
-      PS1+='\[\033[01;37m\]\u/monogon\[\033[01;36m\] \w \$\[\033[00m\] '
+      PS1+='\[\033[01;35m\]\u/monogon\[\033[01;36m\] \w \$\[\033[00m\] '
     fi
     export PS1