fix: download zlib from zlib.net/fossils (root URL now 404s)

zlib.net only serves the current release at the root path; older versions
are moved to the permanent /fossils/ archive. zlib 1.3.2 is now current,
so https://zlib.net/zlib-1.3.1.tar.gz returns 404, which made wget write
no file and the subsequent `tar xzf zlib-1.3.1.tar.gz` fail, breaking the
gocr WASI build.

Point the pinned 1.3.1 download at /fossils/, which is a permanent archive
and won't move again when future zlib versions are released.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: bushidocodes

install_libpng_wasi.sh

@@ -27,7 +27,9 @@ if [ -f "$ZLIB_TARBALL" ]; then
     echo "Tarball $ZLIB_TARBALL already exists. Skipping re-download."
 else
     echo "Tarball $ZLIB_TARBALL does not exist. Downloading the tarball."
-    wget https://zlib.net/$ZLIB.tar.gz
+    # zlib.net only serves the current release at the root; older versions are
+    # permanently archived under /fossils/. (Was https://zlib.net/$ZLIB.tar.gz, now 404.)
+    wget https://zlib.net/fossils/$ZLIB.tar.gz
 fi
 
 # Check if the directory already exists