ruby: set JIT compiler path

[holrock]

ruby-JIT needs callable MJIT_CC path in runtime.

Normally JIT uses CC path with compiled. ruby configure.ac
but lib/spack/spack/env/cc can't call directly.

current:

$ ruby --jit --jit-verbose=1 /tmp/bench.rb
Spack compiler must be run from Spack! Input 'SPACK_ENV_PATH' is missing.
MJIT warning: Making precompiled header failed on compilation. Stopping MJIT worker...

fixed:

$ ruby --jit --jit-verbose=1 /tmp/bench.rb
JIT success (53.3ms): block (4 levels) in <main>@/tmp/bench.rb:9 -> /tmp/_ruby_mjit_p13565u1.c
JIT success (95.1ms): block (3 levels) in <main>@/tmp/bench.rb:8 -> /tmp/_ruby_mjit_p13565u2.c
JIT success (58.6ms): block (5 levels) in <main>@/tmp/bench.rb:16 -> /tmp/_ruby_mjit_p13565u3.c
JIT success (848.0ms): @<internal:kernel>:89 -> /tmp/_ruby_mjit_p13565u4.c
Successful MJIT finish

[alalazo]

That was meant to be fixed in #17141 Will double check this PR. Sorry it went out of our radars.

[becker33]

@alalazo were you able to confirm whether this is still necessary?