Update post install message to show only the source methods that are relevant to your system.

[ryankopf]

When installing Rust on a new system, you get this ugly block of text listing all the possible ways to source the cargo environment variables. However, we already detect which shells are available - so the message should show the ones that exist on the users system.

Instead of:

This is usually done by running one of the following (note the leading DOT):
. "$HOME/.cargo/env"            # For sh/bash/zsh/ash/dash/pdksh
source "$HOME/.cargo/env.fish"  # For fish
source "~/.cargo/env.nu"  # For nushell
source "$HOME/.cargo/env.tcsh"  # For tcsh
. "$HOME/.cargo/env.ps1"        # For pwsh
source "$HOME/.cargo/env.xsh"   # For xonsh

If you have just bash and pwsh, for example, it will instead just show the relevant lines:

This is usually done by running one of the following (note the leading DOT):
. "$HOME/.cargo/env"            # For bash
. "$HOME/.cargo/env.ps1"        # For pwsh