Support sourcing the Spin environment file from a Bash script

At Shopify we want to use TruffleRuby inside other Spin projects, so
it’s important that `source /path/to/truffleruby/.spin/bin/env` works
from those projects’ Bash scripts as well as directly from the command
line.

When the environment file is sourced by another script, `$0` is the name
of the main script being executed rather than the environment file
itself, so `dirname "$0"` will return the (probably incorrect) directory
containing the main script rather than `/path/to/truffleruby/.spin/bin`
as expected.

In Bash, `$BASH_SOURCE` is an array containing the source filenames of
all currently-executing shell functions [0], of which the first is
always the most recent [1], so we can use `$BASH_SOURCE[0]` to directly
ask for the filename of the environment file instead of assuming that
`$0` will contain it.

This change makes `source` work correctly at the cost of relying on a
Bash-specific feature. We also need to support zsh, so support for that
will be added in the next commit.

[0] https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fSOURCE
[1] https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-FUNCNAME

Author: tomstuart

.spin/bin/env

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-SCRIPT_PATH=$(dirname "$0")
+SCRIPT_PATH=$(dirname "$BASH_SOURCE[0]")
 SCRIPT_PATH=$(cd "$SCRIPT_PATH" && pwd)
 TRUFFLERUBY_DIR=$SCRIPT_PATH/../..
 export JT_ENV=jvm-ce