[bug#43976] [PATCH] Chicken build system + some example eggs

[raingloom]

Well, CHICKEN_IMPORT_PATH seems to just be some kind of historical
baggage, at least as far as I can tell. It is not mentioned anywhere on
https://wiki.call-cc.org/man/5 and from what I deciphered from the
sources, the only time it's used it gets prepended to
CHICKEN_REPOSITORY_PATH.

I removed the search path and things still seem to work. It was
probably used in earlier version of Chicken and Nix just never removed
it. Didn't find any related issues either.
I think I'll comment it out for now but leave it in? If there are no
issues with it a few months after merging, it can probably be deleted
entirely...

Possibly more important: I noticed some warnings in some builds about
cp(1). So far it hasn't caused any issues. Gonna fix this, then I think
it'll be mergeable.

```
building srfi-18
   /tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/build-srfi-18
-host -D compiling-extension -J -s -regenerate-import-libraries
-setup-mode -I /tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -C
-I/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -O2 -d1
srfi-18.scm -o
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/srfi-18.so
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/build-srfi-18: line
8: srfi-18.types: Permission denied
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/build-srfi-18
-regenerate-import-libraries -M -setup-mode -static -I
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -emit-link-file
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/srfi-18.link -host
-D compiling-extension -c -unit srfi-18 -D compiling-static-extension
-C -I/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -O2 -d1
srfi-18.scm -o
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/srfi-18.static.o
cp: cannot create regular file 'srfi-18.types': Permission denied
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/build-srfi-18: line
8: srfi-18.types: Permission denied
/gnu/store/aags0k5s6pnk1askg8k3czyhi34gz4pg-chicken-5.2.0/bin/csc
-setup-mode -s -host -I
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -C
-I/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18 -O2 -d0
srfi-18.import.scm -o
/tmp/guix-build-chicken-srfi-18-0.1.5.drv-0/srfi-18/srfi-18.import.so
```