Misleading build error: Fails to find libbpf, even though it is installed

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### What reproduces the bug? Provide code if possible.

```console
❯ cmake -S bpftrace -B build -G Ninja
-- The C compiler identification is GNU 15.2.1
-- The CXX compiler identification is GNU 15.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found ZLIB: /usr/lib/libz.so (found version "1.3.1")
-- Found LibBcc: /usr/lib/libbcc.so
-- Looking for bcc_usdt_foreach
-- Looking for bcc_usdt_foreach - found
-- Found LibBpf: /home/arvid/src/aur/bpftrace-git/src/build/libbpf/include
CMake Error at cmake/FindLibBpf.cmake:85 (message):
  Libbpf version is too old and does not have libbpf_version.h, which was
  introduced in v0.6
Call Stack (most recent call first):
  CMakeLists.txt:104 (find_package)
```

This is very odd because the directory it mentions doesn't exist: 

```console
❯ ls /home/arvid/src/aur/bpftrace-git/src/build/libbpf/include
ls: cannot access '/home/arvid/src/aur/bpftrace-git/src/build/libbpf/include': No such file or directory
```

Also libbpf is newer than 0.6 (it is even a git revision!):

```console
❯ pacman -Qi libbpf-git
Name            : libbpf-git
Version         : 1.6.2.r25.g3f077472-1
[...]
```

And the mentioned header exists:

```console
❯ pacman -Ql libbpf-git   
libbpf-git /usr/
libbpf-git /usr/include/
libbpf-git /usr/include/bpf/
libbpf-git /usr/include/bpf/bpf.h
libbpf-git /usr/include/bpf/bpf_core_read.h
libbpf-git /usr/include/bpf/bpf_endian.h
libbpf-git /usr/include/bpf/bpf_helper_defs.h
libbpf-git /usr/include/bpf/bpf_helpers.h
libbpf-git /usr/include/bpf/bpf_tracing.h
libbpf-git /usr/include/bpf/btf.h
libbpf-git /usr/include/bpf/libbpf.h
libbpf-git /usr/include/bpf/libbpf_common.h
libbpf-git /usr/include/bpf/libbpf_legacy.h
libbpf-git /usr/include/bpf/libbpf_version.h
libbpf-git /usr/include/bpf/skel_internal.h
libbpf-git /usr/include/bpf/usdt.bpf.h
libbpf-git /usr/lib/
libbpf-git /usr/lib/libbpf.a
libbpf-git /usr/lib/libbpf.so
libbpf-git /usr/lib/libbpf.so.1
libbpf-git /usr/lib/libbpf.so.1.7.0
libbpf-git /usr/lib/pkgconfig/
libbpf-git /usr/lib/pkgconfig/libbpf.pc
libbpf-git /usr/share/
libbpf-git /usr/share/doc/
libbpf-git /usr/share/doc/libbpf/
libbpf-git /usr/share/doc/libbpf/README.md
libbpf-git /usr/share/licenses/
libbpf-git /usr/share/licenses/libbpf/
libbpf-git /usr/share/licenses/libbpf/LICENSE.BSD-2-Clause
```

### `bpftrace --info` output
N/A as I can't even build.

Git revision 57c2b5863

Last *known* working revision is c47f16f5, I don't know where in between this broke.

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