fix: add trap to clean up temp directory on unexpected exit

[AnveshKolluri]

Problem

install.sh uses set -e which terminates the script on any command failure.
When this happens (e.g., download 404, network error), the temp directory
created by mktemp -d is never cleaned up — it's leaked in /tmp.

The existing rm -rf "$TMP_DIR" calls only cover manually handled error paths,
but set -e implicit exits bypass all of them.

Repro:

VERSION=v99.99.99 curl -fsSL https://gh.io/copilot-install | bash
# curl fails with 404, set -e kills script, /tmp/tmp.XXXXX left behind


Fix-
Add trap 'rm -rf "$TMP_DIR"' EXIT immediately after mktemp -d. This ensures cleanup on all exit paths. The 4 redundant manual rm
-rf "$TMP_DIR" calls are removed since the trap handles them.

[Copilot]

Pull request overview

Ensures install.sh always cleans up its mktemp -d directory even when set -e causes an implicit/early exit (e.g., failed downloads).

Changes:

• Add an EXIT trap immediately after creating TMP_DIR to remove the temp directory on all exit paths.
• Remove now-redundant manual rm -rf "$TMP_DIR" cleanups from individual error/success branches.

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