fix: make CONFIGFLAGS optional (#5713)

## Issue being fixed or feature implemented
make it possible to run `./contrib/guix/guix-build` without specifying
`CONFIGFLAGS`

## What was done?

## How Has This Been Tested?
run `./contrib/guix/guix-build` w/ and w/out this patch

## Breaking Changes
n/a

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

contrib/guix/libexec/build.sh

@@ -27,13 +27,17 @@ Required environment variables as seen inside the container:
  DIST_ARCHIVE_BASE: ${DIST_ARCHIVE_BASE:?not set}
  DISTNAME: ${DISTNAME:?not set}
  HOST: ${HOST:?not set}
- CONFIGFLAGS: ${CONFIGFLAGS:?not set}
  SOURCE_DATE_EPOCH: ${SOURCE_DATE_EPOCH:?not set}
  JOBS: ${JOBS:?not set}
  DISTSRC: ${DISTSRC:?not set}
  OUTDIR: ${OUTDIR:?not set}
 EOF
 
+cat << EOF
+Optional environment variables as seen inside the container:
+ CONFIGFLAGS: ${CONFIGFLAGS}
+EOF
+
 ACTUAL_OUTDIR="${OUTDIR}"
 OUTDIR="${DISTSRC}/output"