use linux/amd64 platform when running on M1 macs

[zerowidth]

This doesn't work on M1 macs:

$ go run github.com/github/proto-gen-go@v1.1.0
proto-gen-go: building protoc container image...
proto-gen-go: protoc
qemu-x86_64: Could not open '/lib64/ld-linux-x86-64.so.2': No such file or directory
proto-gen-go: protoc command failed: exit status 255
exit status 1

This PR adds --platform=linux/amd64 when it detects it's running on an M1.

After:

$ go run .
proto-gen-go: building protoc container image...
proto-gen-go: protoc
Usage: /usr/local/bin/protoc [OPTION] PROTO_FILES
[...]

An alternative might be trying an arm64-specific base image, but I wasn't able to figure that out.