install.sh: use a more generic SOURCE_STR

SOURCE_STR currently includes the value of $HOME (at install time).
Change SOURCE_STR to include a literal "$HOME" (to be expanded at
profile runtime) so that sourcing nvm will work if the user changes
their username, shares their profile, et cetera.

Also use the more portable single-square-bracket test.
Merge pull request #296 from frasertweedale/fix/generic-source

install.sh: use a more generic SOURCE_STR