build: let additional configure params to be passed into guix (#5705)

## Issue being fixed or feature implemented
Make it possible to pass additional configure params into Guix. This
could be used to setup various sets of nightly/debug builds which could
then be deployed automagically to catch potential issues early.

## What was done?


## How Has This Been Tested?
`CONFIGFLAGS="--enable-debug" HOSTS="x86_64-linux-gnu"
./contrib/guix/guix-build`

## 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/guix-build

@@ -447,6 +447,7 @@ EOF
                                  ${SUBSTITUTE_URLS:+--substitute-urls="$SUBSTITUTE_URLS"} \
                                  ${ADDITIONAL_GUIX_COMMON_FLAGS} ${ADDITIONAL_GUIX_ENVIRONMENT_FLAGS} \
                                  -- env HOST="$host" \
+                                        CONFIGFLAGS="$CONFIGFLAGS" \
                                         DISTNAME="$DISTNAME" \
                                         JOBS="$JOBS" \
                                         SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:?unable to determine value}" \

contrib/guix/libexec/build.sh

@@ -27,6 +27,7 @@ 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: ${DISTSRC:?not set}
@@ -229,7 +230,7 @@ mkdir -p "$OUTDIR"
 ###########################
 
 # CONFIGFLAGS
-CONFIGFLAGS="--enable-reduce-exports --disable-bench --disable-gui-tests --disable-fuzz-binary"
+CONFIGFLAGS+=" --enable-reduce-exports --disable-bench --disable-gui-tests --disable-fuzz-binary"
 case "$HOST" in
     *linux*) CONFIGFLAGS+=" --disable-threadlocal" ;;
 esac