fix(steps-guix): add libunistring and gc builds before guile-2.2.4

steps-guix/gc-8.2.12/pass1.sh

@@ -0,0 +1,16 @@
+# SPDX-FileCopyrightText: 2021 Andrius Štikonas <andrius@stikonas.eu>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+src_prepare() {
+    autoreconf-2.71 -fi
+}
+
+src_configure() {
+    # CFLAGS needed on musl
+    ./configure \
+        --prefix="${PREFIX}" \
+        --libdir="${LIBDIR}" \
+        --disable-shared \
+        CFLAGS='-D_GNU_SOURCE -DNO_GETCONTEXT -DSEARCH_FOR_DATA_START -DUSE_MMAP -DHAVE_DL_ITERATE_PHDR'
+}