iterm: use 'exec' so pane auto-closes with editor
iterm: use 'exec' so pane auto-closes with editor

this uses 'export' to set $TMPDIR instead; fish provides an 'export'
function for bash-compatibility, so this should work for both shells.

note that prior to this change, opening new panes with fish did not
work, as fish does not support `FOO=1 bar` syntax.

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