RFR: Fix jextract GHA [v2]

[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`

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Fix broken `ifOnly`

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/28/files
  - new: https://git.openjdk.java.net/jextract/pull/28/files/32135042..f4e3c74d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=28&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=28&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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