replace go-install with µget, cleanup Makefile

[xrstf]

Summary

This follows our changes in kcp and its related sibling projects. µget is superior to go-install because

• it can handle Go 1.25 compatibility hacks
• can and is optimized for installing non-Go binaries
• it does checksumming
• it can print the resulting binary path
• it can use a system-wide cache, great for when many projects use µget and all of them want golangci-lint
• it doesn't suffer from the NIH syndrome

Like in the other projects, the shell scripts themselves are kept/meant to be usable without make, so they install (using make, however) their dependencies as they see fit.

I also unfuddled with TOOLS_GO_BIN_DIR thingymabob, which seems to have been mostly broken and unusable. From now on, all binaries we produce end up in bin/, all tools we use from external sources are in hack/tools. It made no senses to me to place applyconfiguration-gen in the same folder as the backend binary.

I also took the liberty to move some of the larger chunks of shell code and moved them into actual script files. My brain is bad at reading and writing 20+ line inline bash scripts in Makefiles.

What Type of PR Is This?

/kind cleanup

Release Notes

NONE

[mjudeikis]

@coderabbitai review this please

[coderabbitai]

@mjudeikis I will review the changes in this PR.

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[cnvergence]

I see it is running in kcp for some time already, happy to try it out in the kube-bind