Add Flox as an installation method

[bryanhonof]

kubectx is available and working in Flox.

I did a minimal test as follows.

$ flox init
✨ Created environment 'tmp.OzONlp90TB' (aarch64-darwin)

Next:
  $ flox search <package>    <- Search for a package
  $ flox install <package>   <- Install a package into an environment
  $ flox activate            <- Enter the environment
  $ flox edit                <- Add environment variables and shell hooks

$ flox search kubectx
kubectx                     Fast way to switch between clusters and namespaces in kubectl!
emacsPackages.kubectx-mode  <no description provided>
kubeswitch                          The kubectx for operators

Use 'flox show <package>' to see available versions

$ flox install kubectx
✅ 'kubectx' installed to environment 'tmp.OzONlp90TB'
$ flox activate -- kubectx --version
0.9.5