[interpreter] Set LLVM_DIR for builtin_llvm

[hahnjo]

This allows using the variable to find a system-installed LLVM with builtin_llvm=OFF.

[ferdymercury]

Thanks a lot for the fix.

Unrelated: Wouldn't it be cleaner to define a variable rather than hardcoding in two places the 1 ?

[github-actions]

Test Results

    22 files      22 suites   3d 16h 43m 43s ⏱️
 3 861 tests  3 851 ✅   0 💤 10 ❌
76 285 runs  76 040 ✅ 235 💤 10 ❌

For more details on these failures, see this check.

Results for commit 051effd.

♻️ This comment has been updated with latest results.

[hahnjo]

Unrelated: Wouldn't it be cleaner to define a variable rather than hardcoding in two places the 1 ?

Maybe. Or actually we can live with any minor version, a release x.2.p would signify an ABI-breaking change compared to x.1.0 which is usually not permitted, but we don't really care about. But is you say, it's unrelated and not relevant to this PR so I resolved the conversations.