[libpq][hack] Workaround for libpq

[smklein]

See #213 for full details - this solution is a hack, but it enables building/running on illumos.

[bnaecker]

Looks good to me as a quick fix, but I'd probably get some other comments. Nothing seems like a "good" answer, so this might just be the least bad for the time being. Thanks for doing this!

[jclulow]

FWIW, one could also modify the binaries to include the correct runpath; e.g.,

$ cargo build
   Compiling omicron-nexus v0.1.0 (/ws/omicron/omicron-nexus)
    Finished dev [unoptimized + debuginfo] target(s) in 8.48s

$ target/debug/nexus
ld.so.1: nexus: fatal: libpq.so.5: open failed: No such file or directory
Killed

$ /usr/bin/elfedit -e 'dyn:runpath /opt/ooce/pgsql-13/lib/amd64:/usr/gcc/10/lib/amd64'  target/debug/nexus

$ target/debug/nexus
nexus: parsing arguments: error: The following required arguments were not provided:
    <CONFIG_FILE_PATH>

USAGE:
    nexus [FLAGS] <CONFIG_FILE_PATH>

For more information try --help