Add premier and fireflyk3 to the list of systems where EESSI is available

[julianmorillo]

No description provided.

[boegel]

@julianmorillo You're removing all references to RISC-V with these changes, we should probably keep that, and mention that these different nodes are partitions in a RISC-V Slurm cluster hosted at BSC?

[julianmorillo]

@boegel I removed the references because they referred to riscv.eessi.io which is discontinued. But I agree that mentioning that they are RISC-V clusters is convenient. I'll look into it.