Check shell profile is writeable before modifying

The `set -e` at the top of the script causes the installation to fail to
complete if the shell profile is not writeable. Checking file existence
only is not enough.

(cherry picked from commit ad0dc41899)

scripts/install-nix-from-closure.sh

@@ -92,7 +92,7 @@ p=$NIX_LINK/etc/profile.d/nix.sh
 added=
 for i in .bash_profile .bash_login .profile; do
     fn="$HOME/$i"
-    if [ -e "$fn" ]; then
+    if [ -w "$fn" ]; then
         if ! grep -q "$p" "$fn"; then
             echo "modifying $fn..." >&2
             echo "if [ -e $p ]; then . $p; fi # added by Nix installer" >> $fn