add function to make env vars case-insensitive (#14390)

# Description

This PR adds a new function that allows one to get an env var
case-insensitively. I did this so we can hopefully stop having problems
when Windows has HKLM as path and HKCU as Path.

Instead of just changing every function that used the original one, I
chose the ones that I thought were specific to getting the path. I
didn't want to go all in and make every env get case insensitive, but
maybe we should? 🤷🏻‍♂️

closes #12676

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crates/nu-plugin-engine/src/context.rs

@@ -126,7 +126,7 @@ impl<'a> PluginExecutionContext for PluginExecutionCommandContext<'a> {
     }
 
     fn get_env_var(&self, name: &str) -> Result<Option<&Value>, ShellError> {
-        Ok(self.stack.get_env_var(&self.engine_state, name))
+        Ok(self.stack.get_env_var_insensitive(&self.engine_state, name))
     }
 
     fn get_env_vars(&self) -> Result<HashMap<String, Value>, ShellError> {