feat: Add --profile-env flag to exec command to facilitate AWS_PROFILE SDK usage

[tthayer]

I have some Pulumi stacks that depend on the AWS_PROFILE env var and cached sso token to run. Typically this involves me having to run aws sso login MYPROFILE and then setting the env var manually or doing something with a script. I'm very invested in using aws-vault for my workflows since it works great for everything else I have to do that isn't necessarily Pulumi-related.
I have a (very heavily robot-coded) branch that I have been using for this purpose today that adds the --profile-env flag to the exec command and it does the appropriate thing for my use-case without breaking normal exec usage.

#340 Has been created to implement this.

[mbevc1]

Thanks @tthayer for your contribution! I think it's tangent to discussion we had in #133 😄 . But there we were thinking to include AWS_PROFILE in exec and current behaviour seems intentional -

aws-vault/cli/exec.go

Line 260 in 94e3e0a

env.Unset("AWS_PROFILE")

(not sure about historical context there), but without SSO. Happy to review your PR once tests go green 👍