enhancement: Make install.sh more reliable

[nabil1440]

Blocker: no. This can wait. Do fix step 6 in the same release as the monitoring agent (#26 to #30).

Defect

• The script does not use set -euo pipefail.
• curl -s does not use -f. On a 404 error, the script saves the error page as the tarball. Then, the script shows "Failed to extract". This message does not tell the real cause.
• The script runs apt-get install grep without apt-get update. GNU grep on Ubuntu always supports -P. Thus, this step is not necessary.
• When the script stops because of a rate limit, it does not remove its temporary file.
• The script uses grep to parse JSON.
• The script does not compare the download with a checksum.
• On a server with the monitoring agent, /usr/local/bin/fly is a link to ~fly/.fly/bin/fly. The script moves the new binary to /usr/local/bin/fly. mv replaces the link with a separate file.

Cost

• Some errors show an incorrect cause. Thus, installation problems are hard to find.
• A command that fails does not always stop the script.
• On a server with the agent, the server then has two binaries. fly update updates the separate file, and the agent continues with the old binary.

Benefit of the fix

• Each error stops the script with a correct message.
• The script does not leave temporary files.
• The script examines the download before it installs the binary (with enhancement: Examine release downloads with checksums #9).
• On a server with the agent, the CLI and the agent use the same binary.

Fix

1. Add set -euo pipefail.
2. Use curl -fsSL for each download.
3. Remove the grep -P installation step.
4. Add a trap that removes the temporary files.
5. Compare the download with checksums.txt (refer to enhancement: Examine release downloads with checksums #9). ci: run make check on every change and publish releases with gh #25 publishes it.
6. If /usr/local/bin/fly is a link, install to the target of the link, and keep the owner of the target.

Why this can wait

• The script operates correctly on Ubuntu at this time.
• The FlyWP installer, not this script, installs the agent. Fix step 6 is necessary only when a person runs this script on a server with the agent.