Allow rootdir and prefix to be modified for `just install`

[ryanabx]

I think these are the changes necessary to allow rootdir and prefix to be changed when running just install

Without this PR, when I try to run just rootdir=/build install the following error occurs:

error: Variable `rootdir` overridden on the command line but not present in justfile

This is necessary for my custom build artifacts of COSMIC, which I'm currently using for my atomic images

[mmstick]

Since the install recipe defines these arguments, you could run just install rootdir=/build.

[ryanabx]

Since the install recipe defines these arguments, you could run just install rootdir=/build.

I had some weird issues with doing that:

My command was just install rootdir=/build prefix=/usr and it didn't install things where I expected...

When compiling and installing https://github.com/pop-os/launcher, or using any of the individual justfiles for cosmic's components, the issues don't occur, and I run just rootdir=/build prefix=/usr install for those

[Drakulix]

Arguments are not named. You can just do just install /build /usr

[ryanabx]

Arguments are not named. You can just do just install /build /usr

Thanks! I'm new to just, I'll give it a try