Show loading icon in the environments dropdown when environments query is still loading

Follow-up for #388952 (closed).

The following discussion from !113171 (merged) should be addressed:

• @mgandres started a discussion: (+4 comments)

  Removed this because that means the variables table will show the loading icon while searching for environments (notice the background during the search):

  Screen_Recording_2023-03-09_at_20.43.35

We can instead use the loading state of the dropdown when the component is loading the environments query for the first time:

This should not apply to subsequent refetches, since that would be the equivalent of searching using a new search term. The dropdown should be available during a search, with searching = true to show the loading icon in the dropdown contents instead (which we already do).