docs: Mention arm64 for ARCH

[jubr]

Description

Hola, thanks for Task, it's awesome!

Perhaps worth also mentioning arm64 explicitly in docs #task-functions for ARCH, now it only shows arm.

grtz, jubr

Version

v3.40.1

Operating system

arm64

Experiments Enabled

No response

Example Taskfile

tasks:
  test:
    vars:
      RUNTIME_ARCH: 'linux/{{ .ARCH }}'