rbm gives an error if a build script contains a wide character

When a build script contains a wide character, rbm fails with the following error:

Error: Template Error:
undef error - Wide character in null operation at input text line 1.

The reason is that in var/build_id, we call sha256 on the content of the build script, which is using the sha256_hex function, which doesn't support wide characters: http://perldoc.perl.org/Digest/SHA.html#UNICODE-AND-SIDE-EFFECTS

We currently don't use any build script with wide character in tor-browser-build. However the file projects/tor-browser/RelativeLink/start-tor-browser does contain a wide character (curly quotes: ”), which is a problem if we want to use it as a template (for #21998 (moved)).