testing/git-absorb: disable on riscv64

https://build.alpinelinux.org/buildlogs//build-edge-riscv64/testing/git-absorb/git-absorb-0.6.11-r0.log

Multiple:

  warning: gcc: error: '-march=rv64': first ISA subset must be 'e', 'i' or 'g'

Could perhaps be patched to link against system libs, like community/wezterm, or use some flag or variable to set march to rv64g.