[NOT WORKING] CI: Add gfortran 15 on Linux

[bonachea]

@rouson Here is my failed attempt to build Matcha on Linux ubuntu-latest with gfortran 15.2.0, OpenCoarrays 2.10.3 and the OC's default MPICH 3.2.

The link errors I mentioned today look like this:

/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: build/caf_33623D7BE955BBCD/matcha/libmatcha.a(src_matcha_subdomain_s.F90.o): warning: relocation against `__caf_send_to_remote_fn_index_5.10' in read-only section `.text'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: build/caf_33623D7BE955BBCD/matcha/libmatcha.a(src_matcha_subdomain_s.F90.o): in function `__subdomain_m_MOD_assign_and_sync':
/home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:161:(.text+0x82cd): undefined reference to `__caf_send_to_remote_fn_index_2.13'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:161:(.text+0x82eb): undefined reference to `__caf_send_to_remote_fn_index_2.13'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:162:(.text+0x863e): undefined reference to `__caf_send_to_remote_fn_index_3.12'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:162:(.text+0x865c): undefined reference to `__caf_send_to_remote_fn_index_3.12'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: build/caf_33623D7BE955BBCD/matcha/libmatcha.a(src_matcha_subdomain_s.F90.o): in function `__subdomain_m_MOD_define':
/home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:66:(.text+0xf045): undefined reference to `__caf_send_to_remote_fn_index_4.11'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:66:(.text+0xf062): undefined reference to `__caf_send_to_remote_fn_index_4.11'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:67:(.text+0xf3a7): undefined reference to `__caf_send_to_remote_fn_index_5.10'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: /home/runner/work/matcha/matcha/././src/matcha/subdomain_s.F90:67:(.text+0xf3c4): undefined reference to `__caf_send_to_remote_fn_index_5.10'
/home/linuxbrew/.linuxbrew/Cellar/binutils/2.45.1/bin/ld: warning: creating DT_TEXTREL in a PIE

I get very similar link failures if I install MPICH 4.3.2 and use that instead (using this commit)

I don't know how to fix this problem and this is the reason I was unable to add GCC 15.x testing to Matcha.