Add ability to override arch with env var

[danieldides]

Workaround for #19

Inspired by the way the golang plugin handles this, this PR will allow the user to set an environment variable ASDF_KUBECTL_OVERWRITE_ARCH and specify an alternative architecture to download the binary from.

Mac users on M1 laptops will have issues downloading older versions of kubectl that do not have arm64 builds. Their installations right now silently fail and there is no way to configure asdf to download the amd64 build instead.

Demonstration (M1 Max):

% uname -m
arm64

% kubectl version
No preset version installed for command kubectl
Please install a version by running one of the following:

asdf install kubectl 1.18.17

% asdf install kubectl 1.18.17

Downloading kubectl from https://storage.googleapis.com/kubernetes-release/release/v1.18.17/bin/darwin/arm64/kubectl

% kubectl version
No preset version installed for command kubectl

% ASDF_KUBECTL_OVERWRITE_ARCH=amd64 asdf install kubectl 1.18.17
Downloading kubectl from https://storage.googleapis.com/kubernetes-release/release/v1.18.17/bin/darwin/amd64/kubectl

% kubectl version
Client Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.17", GitCommit:"68b4e26caf6ede7af577db4af62fb405b4dd47e6", GitTreeState:"clean", BuildDate:"2021-03-18T01:02:41Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"darwin/amd64"}

[ryancabanas]

Thanks for this! This might be a good conversation to link to over at the README's Architecture Override section to help explain how to properly use it, for noobs like me! 😁 Just a thought. Thanks for your work!