bug#32293: wrap-program with non-colon separtor produces incorrect bash substitutions

[Jelle Licht]

Hi all,

While working to package some lua-related stuff, I need to deal with the
LUA_PATH environment variable. This variable uses `;' as a separator, and
as such, I wanted to wrap one of my programs

--8<---------------cut here---------------start------------->8---
...
(wrap-program (string-append out "/bin/fennel")
                   `("LUA_PATH" ";" prefix (,path)))
...
--8<---------------cut here---------------end--------------->8---

... which gives me the following snippet for the fennel script:

--8<---------------cut here---------------start------------->8---
export 
LUA_PATH="/gnu/store/3yjzvzwczi37snccrxbw7xsmbns1qc7a-fennel-1-1.f2a3d3b/share/lua/5.3/?.lua${LUA_PATH;+;}$LUA_PATH"
--8<---------------cut here---------------end--------------->8---

... which is not a correct bash substitution. I _think_ the first `;' in
`{LUA_PATH;+;}' needs to be a colon instead, at least if `wrap-program'
is only used to generate bash-compliant wrappers.

It seems this can be (easily?) fixed around guix/build/utils.scm:1055,
but any changes I made there had me starting to build bash and other
things from scratch.