Update switch environment file from env_hook

If the environment file is missing, recomputing it can take a noticeable
amount of time which delays every single return of the prompt when the
env_hook is enabled.

The use of atomic write on the environment file instead of a lock means
that the environment hooks can store the generated environment file if
it's missing, or needs regenerating.

src/state/shellscripts/env_hook.csh

@@ -1 +1 @@
-alias precmd 'eval `opam env --shell=csh --readonly`'
+alias precmd 'eval `opam env --shell=csh`'

src/state/shellscripts/env_hook.fish

@@ -1,3 +1,3 @@
 function __opam_env_export_eval --on-event fish_prompt
-    eval (opam env --shell=fish --readonly 2> /dev/null)
+    eval (opam env --shell=fish 2> /dev/null)
 end

src/state/shellscripts/env_hook.sh

@@ -1,6 +1,6 @@
 _opam_env_hook() {
  local previous_exit_status=$?;
- eval $(opam env --shell=bash --readonly 2> /dev/null <&- );
+ eval $(opam env --shell=bash 2> /dev/null <&- );
  return $previous_exit_status;
 };
 if ! [[ "$PROMPT_COMMAND" =~ _opam_env_hook ]]; then

src/state/shellscripts/env_hook.zsh

@@ -1,5 +1,5 @@
 _opam_env_hook() {
-    eval $(opam env --shell=zsh --readonly 2> /dev/null <&-);
+    eval $(opam env --shell=zsh 2> /dev/null <&-);
 }
 typeset -ag precmd_functions;
 if [[ -z ${precmd_functions[(r)_opam_env_hook]} ]]; then