Add -j flag to linux-build.bash so that builds are done in parallel

[uwagura]

This PR adds the -j flag to $makeflags, so that all builds are now done in parallel. In my tests on gaea, this cut the model build time by more than half:
Without the -j flag:

135759991                                   MOM6SIS2_gaea_build   gfdl_med        128  COMPLETED   00:17:20 

With the -j flag:

135761241                          MOM6SIS2_gaea_parallel_build   gfdl_med        128  COMPLETED   00:06:58

[uwagura]

The effect is more subtle on the CI than on gaea. For this build with the -j flag, the build times were

build_mom6sis2_debug: 3 min 16s
build_mom6solo_repro: 7 min 30s

Whereas for the last build:

build_mom6sis2_debug: 3 min 54s
build_mom6solo_repro: 9 min 30s

will look into this before merging @theresa-cordero

[uwagura]

Latest run with parallel build of libyaml is even faster:

build_mom6sis2_debug: 2 min 39s
build_mom6solo_repro: 6 min 34s

I think the reason why I'm not seeing the > 50% speed up I saw with the full repro build on gaea is 1.) that second build may not have included libyaml ( which does not seem to be rebuilt if the .libs directory is present ) and 2.) building cobalt serially in repro mode is a bigger bottleneck during that build process than serialization is to the mom6sis2_debug build or the mom6solo_repro build.

Likewise, the time differences in the mom6sis2 and mom6solo builds can likely be explained by the fact that one is built in debug mode, and one is built in repro.

[theresa-cordero]

Is it safe to do a squash merge here or should we keep two commits?

[uwagura]

I think a squash merge should be fine