Action fails with a vague error if no phpunit version specified in YAML or composer.json/lock - phpunit: line 2: syntax error: unexpected newline

[peternewman]

These changes:
https://github.com/php-actions/example-phpunit/compare/master...peternewman:example-phpunit:peternewman-test-breaking?expand=1

Are sufficient to generate

/usr/local/bin/phpunit: line 2: syntax error: unexpected newline
Error: Process completed with exit code 2.

Rather than say using the latest version.

Confusingly, /usr/local/bin/phpunit exists at this point, and appears to have valid code in it (unless this is local, rather than within Docker or something

[peternewman]

I guess possibly arises from #54