Apple Script can't find executables

Unless you've done something highly creative, Script Editor is using sh to run shell scripts. It does not use your configured shell.

That points to a copy of bash by default.

When bash is invoked (as a login shell) as sh, it only reads /etc/profile and ~/.profile. But it reads no startup files otherwise. So it is not reading any shell configuration you think you are providing.

So you should be just inheriting from the system. Note that pbpaste is findable since it's in /usr/bin and the default path includes /usr/bin.

Since that doesn't work, something else has been changed.