nix: Add hashbang for scripts.

[[ isn't actually available in the system's default shell.

Fixes error: [[: not found.

Also adds default bash safeguards.

gdb/default.nix

@@ -25,10 +25,16 @@ with nixpkgs; rec {
   };
 
   run-ghc-gdb = writeScriptBin "ghc-gdb" ''
+    #!${pkgs.bash}/bin/bash
+    set -eu -o pipefail
+
     ${gdb}/bin/gdb -x ${gdbinit}/gdbinit "$@"
   '';
 
   run-ghc-rr = writeScriptBin "ghc-rr" ''
+    #!${pkgs.bash}/bin/bash
+    set -eu -o pipefail
+
     args="$@"
     if [[ "$1" == "replay" ]]; then
       args="$args --debugger ${gdb}/bin/gdb -x ${gdbinit}/gdbinit"
@@ -82,6 +88,9 @@ with nixpkgs; rec {
 
   # useful to render `ghc closure-deps` output
   dot2svg = writeScriptBin "dot2svg" ''
+    #!${pkgs.bash}/bin/bash
+    set -eu -o pipefail
+
     if [[ $# == 0 ]]; then
       echo "Usage: $0 [dot file]"
       exit 1