Accept the shell argument for fish

[rgbkrk]

Rely on --shell in the same way Windows users deal with.

I made this use $SHELL at first then realized an inconsistency with the other
tooling. Would it be ok to make --shell the primary choice with a fallback
to whatever $SHELL is defined as to determine the script?

kyle@lack ~/c/s/g/g/carina> make; and ./carina env test2 --shell fish
go get ./...
CGO_ENABLED=0 go build -a -tags netgo -ldflags '-w -X github.com/getcarina/carina/version.Commit=f0818b51ab726075b500a11fd079a9c20eb5a1de -X github.com/getcarina/carina/version.Version=v1.3.0-8-gf0818b5' -o carina .
source /Users/rgbkrk/.carina/clusters/rgbkrk/test2/docker.fish
# Run the command below to get your Docker environment variables set:
# eval $(carina env test2)

[rgbkrk]

/cc @carolynvs