fix(cli): do not create bin folder within DENO_INSTALL_ROOT

[mahtaran]

As can be seen in the documentation, DENO_INSTALL_ROOT is meant to be the folder wherein Deno will install executables, with the default being $HOME/.deno/bin.

However, the current behaviour of the install and uninstall commands, if DENO_INSTALL_ROOT is set, is to still append "bin" to the path. This leads to executables being installed in a folder like $HOME/.deno/bin/bin, which is obviously not the intended behaviour.

This commit removes this appending. Do note that this is a breaking change in the behaviour of the --root flag, as previously it referred to a folder wherein the bin folder was located, but now it refers to the bin folder itself. For example: deno install --root /foo/bar test.ts would previously install the executable in /foo/bar/bin/test, but now it will install it in /foo/bar/test.

This would also most certainly break many third-party tools that expect the current behaviour. See for example:

• https://github.com/jdx/mise/blob/0cea93b27bc46ba9219557532d6d1d061d05f81d/src/plugins/core/deno.rs#L144
• https://github.com/asdf-community/asdf-deno/blob/13f7f08c93989fdc0b372e78df226e511fc13fa9/bin/exec-env#L7

Closes #26442

Please do note that, as I am currently on a family visit out of the country, I currently only have my poor little laptop. As such, I do not have the storage capacity to build the project or run the test suite, so THIS CODE IS UNTESTED. If someone else could test it for me, that'd be grand, and otherwise I'll do it when I'm back home in a couple of weeks.

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[mahtaran]

I seem to have completely missed that @dovakin0007 already submitted a fix in #26446, apologies! I do believe this PR is more 'correct' as it modifies the behaviour to be what the documentation says it is, however Gowtham's PR will cause noticeably less (but likely still some) breaking of third-party tools and existing set-ups. I'll leave the choice on what's best up to the maintainers.