guix: Register garbage collector root for containers

By registering the container profiles as garbage collector roots, it
will prevent `guix gc` from garbage collecting derivations which our
container needs and inconvieniencing the user with a rebuild.

contrib/guix/guix-build

@@ -207,7 +207,7 @@ time-machine() {
 # Precious directories are those which should not be cleaned between successive
 # guix builds
 depends_precious_dir_names='SOURCES_PATH BASE_CACHE SDK_PATH'
-precious_dir_names="${depends_precious_dir_names} OUTDIR_BASE"
+precious_dir_names="${depends_precious_dir_names} OUTDIR_BASE PROFILES_BASE"
 
 # Usage: contains IFS-SEPARATED-LIST ITEM
 contains() {
@@ -275,6 +275,14 @@ outdir_for_host() {
     echo "${OUTDIR_BASE}/${1}"
 }
 
+# Usage: profiledir_for_host HOST COMMAND
+#
+#   HOST: The current platform triple we're building for
+#
+profiledir_for_host() {
+    echo "${PROFILES_BASE}/${2}-${1}"
+}
+
 
 #########
 # BUILD #
@@ -404,6 +412,7 @@ EOF
                                  --keep-failed \
                                  --fallback \
                                  --link-profile \
+                                 --root="$(profiledir_for_host "${HOST}" build)" \
                                  ${SUBSTITUTE_URLS:+--substitute-urls="$SUBSTITUTE_URLS"} \
                                  ${ADDITIONAL_GUIX_COMMON_FLAGS} ${ADDITIONAL_GUIX_ENVIRONMENT_FLAGS} \
                                  -- env HOST="$host" \

contrib/guix/libexec/prelude.bash

@@ -61,3 +61,6 @@ OUTDIR_BASE="${OUTDIR_BASE:-${VERSION_BASE}/output}"
 
 var_base_basename="var"
 VAR_BASE="${VAR_BASE:-${VERSION_BASE}/${var_base_basename}}"
+
+profiles_base_basename="profiles"
+PROFILES_BASE="${PROFILES_BASE:-${VAR_BASE}/${profiles_base_basename}}"