Use absolute path /bin/sh instead of relying on the PATH environment variable.

[v1nc3n4]

Actually, if the sh command is not accessible through the PATH environment variable, or if another executable with the same name has priority in the environment variable, running any frontend script would fail: the shell would not be found, which is required to launch node/npm/npx/yarn executables in a dedicated shell process.

To avoid such - very rare - case, the plugin must rely on the absolute path /bin/sh.

This issue was identified after a build under Travis with Ubuntu Bionic failed for this exact reason: the sh command was not found using the PATH environment variable.