Fix linux install command

[finestructure]

I'm getting an error running the swiftly install command in a docker container (ubuntu:jammy, although I don't think that matters):

$ curl -O https://download.swift.org/swiftly/linux/swiftly-$(uname -m).tar.gz && \
tar zxf swiftly-$(uname -m).tar.gz && \
./swiftly init --quiet-shell-followup && \
. "${SWIFTLY_HOME_DIR:-~/.local/share/swiftly}/env.sh" && \
hash -r
...
The global default toolchain has been set to `Swift 6.1.2`
Swift 6.1.2 installed successfully!
/bin/sh: 1: .: cannot open ~/.local/share/swiftly/env.sh: No such file

I believe the tail end of the install command should be

. "${SWIFTLY_HOME_DIR:-$HOME/.local/share/swiftly}/env.sh"

to avoid that error.