RFR: Fix jextract GHA

[Maurizio Cimadamore]

There are a couple of issue with the jextract GHA:
* the action does not ignore pr/* branches, which seems to be the norm in JDK
* the build is still referring to the old variable name `libclang_home` instead of `llvm_home`

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Commit messages:
 - Fix jextract GHA

Changes: https://git.openjdk.java.net/jextract/pull/28/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=28&range=00
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jextract/pull/28.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/28/head:pull/28

PR: https://git.openjdk.java.net/jextract/pull/28