Fix Go 1.22 Docker image build failure
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:

```sh
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.