installation: allow profile modification to be skipped (#1072)

The current behaviour modifies the first writeable file from amongst
.bash_profile, .bash_login and .profile.  So .bash_profile (if it is
writable) would be modified even if a user has already sourced nix.sh
in, say, .profile.

This commit introduces a new environment variable,
NIX_INSTALLER_NO_MODIFY_PROFILE.  If this is set during installation,
then the modifications are unconditionally skipped.

This is useful for users who have a manually curated set of dotfiles
that they are porting to a new machine. In such scenarios, nix.sh is
already sourced at a place where the user prefers.  Without this
change, the nix installer would insist on modifying .bash_profile if
it exists.

This commit also add documentations for both the current behaviour and
the new override.

doc/manual/installation/installing-binary.xml

@@ -26,6 +26,14 @@ $ mkdir /nix
 $ chown alice /nix
 </screen>
 
+The install script will modify the first writable file from amongst
+<filename>.bash_profile</filename>, <filename>.bash_login</filename>
+and <filename>.profile</filename> to source
+<filename>~/.nix-profile/etc/profile.d/nix.sh</filename>. You can set
+the <command>NIX_INSTALLER_NO_MODIFY_PROFILE</command> environment
+variable before executing the install script to disable this
+behaviour.
+
 </para>
 
 <!--

scripts/install-nix-from-closure.sh

@@ -89,10 +89,12 @@ if [ -z "$_NIX_INSTALLER_TEST" ]; then
     $nix/bin/nix-channel --update nixpkgs
 fi
 
+added=
+if [ -z "$NIX_INSTALLER_NO_MODIFY_PROFILE" ]; then
+
     # Make the shell source nix.sh during login.
     p=$HOME/.nix-profile/etc/profile.d/nix.sh
 
-added=
     for i in .bash_profile .bash_login .profile; do
         fn="$HOME/$i"
         if [ -w "$fn" ]; then
@@ -105,6 +107,8 @@ for i in .bash_profile .bash_login .profile; do
         fi
     done
 
+fi
+
 if [ -z "$added" ]; then
     cat >&2 <<EOF