RFR: 8302879: doc/building.md update link to jtreg builds

[George Adams]

The Eclipse Adoptium project recently moved its CI from ci.adoptopenjdk.net to ci.adoptium.net. The link to jtreg builds needs updating

See https://github.com/adoptium/infrastructure/issues/2932 for more context

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Commit messages:
 - 8302879: doc/building.md update link to jtreg builds

Changes: https://git.openjdk.org/jdk/pull/12665/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12665&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302879
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12665.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12665/head:pull/12665

PR: https://git.openjdk.org/jdk/pull/12665