Improve issue reporting links

[dg0yt]

Adds link to search known issues, in order to avoid duplicates. e.g.
https://github.com/microsoft/vcpkg/issues?q=is%3Aissue+is%3Aopen+in%3Atitle+cpr

Sets package name and template for reporting build errors. The links are somewhat ugly, but clickable in VS Code and in many terminals. E.g.
https://github.com/microsoft/vcpkg/issues/new?template=report-package-build-failure.md&title=[cpr]+Build+error

Full output:

Error: Building package cpr:x64-linux failed with: BUILD_FAILED
Please ensure you're using the latest portfiles with `git pull` and `./vcpkg update`.
Then check for known issues at:
  https://github.com/microsoft/vcpkg/issues?q=is%3Aissue+is%3Aopen+in%3Atitle+cpr
You can submit a new issue at:
  https://github.com/microsoft/vcpkg/issues/new?template=report-package-build-failure.md&title=[cpr]+Build+error
including:
  package: cpr[core]:x64-linux -> 1.6.2#1
  vcpkg version: cbc22a396 2021-11-17 (13 hours ago)
  vcpkg-tool version: 2021-11-15-unknownhash-debug

Additionally, attach any relevant sections from the log files above.