add update-rtools option

[jeroen]

@ijlyttle @yutannihilation @clauswilke this should fix your immediate issue with rust.

Introduces an option to update rtools to the latest builds of gcc and libraries. This is mainly useful for the preinstalled rtools40 installations on the GHA image.

[jimhester]

If this option is set then we would not need to also download and install rtools separately right? Since the only time you would use this would be updating the rtools now included in the default image

[jeroen]

In practice we never need to download and install rtools because it is always preinstalled.

But even when we would, it can still be useful to update it because the installer on CRAN gets updated rarely because changing the sha256 breaks tools like chocolatey which check for integrity.

[yutannihilation]

Thanks for this, Jeroen!

@ijlyttle @yutannihilation @clauswilke this should fix your immediate issue with rust.

Btw, maybe you mistyped @Ilia-Kosenkov?

[jimhester]

Thanks!