read old index.pidx to obtain URL before using the default

[bgn42]

Fixes

• try first the URL of existing index.pidx before using the default at keil.com

Changes

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).

[qltysh]

Diff Coverage: The code coverage on the diff in this pull request is 30.0%.

Total Coverage: This PR will decrease coverage by 0.06%.

File Coverage Changes

Path	File Coverage Δ	Indirect
cmd/installer/root.go	-0.2

🛟 Help

• Diff Coverage: Coverage for added or modified lines of code (excludes deleted files). Learn more.

• Total Coverage: Coverage for the whole repository, calculated as the sum of all File Coverage. Learn more.

• File Coverage: Covered Lines divided by Covered Lines plus Missed Lines. (Excludes non-executable lines including blank lines and comments.)

  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.

[jkrech]

LGTM