Don’t source bashrc in pure mode

Pure mode should not try to source the user’s bashrc file. These may
have many impurities that the user does not expect to get into their
shell.

Fixes #3090

src/nix-build/nix-build.cc

@@ -412,7 +412,7 @@ static void _main(int argc, char * * argv)
         auto rcfile = (Path) tmpDir + "/rc";
         writeFile(rcfile, fmt(
                 (keepTmp ? "" : "rm -rf '%1%'; "s) +
-                "[ -n \"$PS1\" ] && [ -e ~/.bashrc ] && source ~/.bashrc; "
+                (pure ? "" : "[ -n \"$PS1\" ] && [ -e ~/.bashrc ] && source ~/.bashrc;")
                 "%2%"
                 "dontAddDisableDepTrack=1; "
                 "[ -e $stdenv/setup ] && source $stdenv/setup; "