Fix Go 1.22 Docker image build failure

[generalmimon]

See https://github.com/kaitai-io/kaitai_struct_docker_images/actions/runs/7989563895/job/21816399533#step:4:122:

#13 [8/9] RUN ./prepare
#13 0.211 go: modules disabled by GO111MODULE=off; see 'go help modules'
#13 ERROR: process "/bin/sh -c ./prepare" did not complete successfully: exit code: 1
------
 > [8/9] RUN ./prepare:
0.211 go: modules disabled by GO111MODULE=off; see 'go help modules'
------
Dockerfile:20
--------------------
  18 |     
  19 |     # Install rest of go-specific dependencies
  20 | >>> RUN ./prepare
  21 |     
  22 |     # Validate that we've got correct version, e.g. something like this:
--------------------
ERROR: failed to solve: process "/bin/sh -c ./prepare" did not complete successfully: exit code: 1
Error: Process completed with exit code 1.

See https://github.com/kaitai-io/kaitai_struct_docker_images/blob/ef0ad6e321d86839c7a929a5d5221bfcf1099531/src/go/_common/prepare#L3:

GO111MODULE=off go get -v github.com/kaitai-io/kaitai_struct_go_runtime/kaitai

The reason this error suddenly appears in Go 1.22 (and not in Go 1.21) is explained at https://tip.golang.org/doc/go1.22#go-command:

go get is no longer supported outside of a module in the legacy GOPATH mode (that is, with GO111MODULE=off). Other build commands, such as go build and go test, will continue to work indefinitely for legacy GOPATH programs.

So apparently we need to change the way we install test dependencies in order to run the test suite on Go 1.22 and later.