Support for RISCv64gc

[NeilHanlon]

Is your feature request related to a problem? Please describe.

I tried to install direnv from script on a risc-v machine and received an error:

[installer] Machine riscv64 not supported by the installer.\n Go to https://direnv for alternate installation methods.
[installer] the script failed with error 1.\n \n To report installation errors, submit an issue to\n     https://github.com/direnv/direnv/issues/new/choose

Describe the solution you'd like

Support RISC-V in the installer.

Describe alternatives you've considered

N/a

Additional context

Fedora's distro packaging for direnv on risc-v works great!