fix: Respect globally set SDKMAN_DIR

Also, print a warning during shell startup
if a custom install path is set, but
SDKMAN! is not installed there.

fixes #52

test/features/support/helpers.rb

@@ -64,10 +64,12 @@ def run_fish_command(cmd)
     end.to_h
 
     out, status = Open3.capture2e(<<~FISH
+      #{@command.nil? ? '' : @command}
       fish -c '#{cmd} > #{files[:stdout]} 2> #{files[:stderr]}; \
                echo $status > #{files[:status]}; \
                env > #{files[:env]}; \
               '
+      #{@command.nil? ? '' : ')'}
     FISH
     )