Zstd compress all images

I tried gzip, bzip2, xz and xzstd. Xz 22s for a 128MB disk image. Out of
all other options zstd was both the fastest to compress (<1s) and
produced the smallest file (28MB).

.gitlab-ci.yml

@@ -21,7 +21,8 @@ before_script:
         nasm \
         pkg-config \
         texinfo \
-        wget &&
+        wget \
+        zstd &&
     curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain none
 
 img: