NA: Use relative path for NA plugin search

[jolivier23]

Calculate relative path at build time and use it at runtime to find the plugin directory

[github-actions]

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[jolivier23]

I have read the CLA Document and I hereby sign the CLA

[soumagne]

just a few comments but looks good to me otherwise, thanks!

[soumagne]

See also this warning from the CI: https://cdash.mercury.daos.io/viewBuildError.php?type=1&buildid=2910

/home/runner/work/mercury/mercury/src/na/na.c: In function 'resolve_plugin_path':
[CTest: warning matched] /home/runner/work/mercury/mercury[na/na.c:279](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/na/na.c#L279):17: warning: ISO C forbids passing argument 1 of 'dladdr' between function pointer and 'void *' [-Wpedantic]
  279 |     if (!dladdr(resolve_plugin_path, &info)) {
      |                 ^~~~~~~~~~~~~~~~~~~
In file included from /home/runner/work/mercury/mercury[util/mercury_dl.h:18](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/util/mercury_dl.h#L18),
                 from /home/runner/work/mercury/mercury[na/na.c:19](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/na/na.c#L19):
[CTest: warning matched] [include/dlfcn.h:98](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/include/dlfcn.h#L98):32: note: expected 'const void *' but argument is of type 'char * (*)(const char *)'
   98 | extern int dladdr (const void *__address, Dl_info *__info)
      |                    ~~~~~~~~~~~~^~~~~~~~~

[jolivier23]

See also this warning from the CI: https://cdash.mercury.daos.io/viewBuildError.php?type=1&buildid=2910

/home/runner/work/mercury/mercury/src/na/na.c: In function 'resolve_plugin_path':
[CTest: warning matched] /home/runner/work/mercury/mercury[na/na.c:279](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/na/na.c#L279):17: warning: ISO C forbids passing argument 1 of 'dladdr' between function pointer and 'void *' [-Wpedantic]
  279 |     if (!dladdr(resolve_plugin_path, &info)) {
      |                 ^~~~~~~~~~~~~~~~~~~
In file included from /home/runner/work/mercury/mercury[util/mercury_dl.h:18](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/util/mercury_dl.h#L18),
                 from /home/runner/work/mercury/mercury[na/na.c:19](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/na/na.c#L19):
[CTest: warning matched] [include/dlfcn.h:98](https://github.com/mercury-hpc/mercury/blob/8f30b7be1d8fd6b0bf99c2f572dc9b277661c44b/include/dlfcn.h#L98):32: note: expected 'const void *' but argument is of type 'char * (*)(const char *)'
   98 | extern int dladdr (const void *__address, Dl_info *__info)
      |                    ~~~~~~~~~~~~^~~~~~~~~

hopefully it's fixed with the cast