build.sh: Fix overquoting for ${cmd}

Fixes:
```
$ ./build.sh

This build script is using Podman to run the build in an isolated environment.

antora: option '--html-url-extension-style <choice>' argument 'indexify site.yml' is invalid. Allowed choices are default, drop, indexify.
```

See https://pagure.io/fedora-docs/template/pull-request/21

build.sh

@@ -10,7 +10,7 @@ if uname | grep -iwq darwin; then
     echo ""
     echo "This build script is using Docker container runtime to run the build in an isolated environment."
     echo ""
-    docker run --rm -it -v "$(pwd):/antora" "${image}" "${cmd}"
+    docker run --rm -it -v "$(pwd):/antora" "${image}" ${cmd}
 
 elif uname | grep -iq linux; then
     # Running on Linux.
@@ -26,22 +26,22 @@ elif uname | grep -iq linux; then
         echo ""
         echo "This build script is using Podman to run the build in an isolated environment."
         echo ""
-        podman run --rm -it -v "$(pwd):/antora:z" "${image}" "${cmd}"
+        podman run --rm -it -v "$(pwd):/antora:z" "${image}" ${cmd}
 
     elif [ -f /usr/bin/docker ]; then
         echo ""
         echo "This build script is using Docker to run the build in an isolated environment."
         echo ""
 
         if groups | grep -wq "docker"; then
-            docker run --rm -it -v "$(pwd):/antora:z" "${image}" "${cmd}"
+            docker run --rm -it -v "$(pwd):/antora:z" "${image}" ${cmd}
         else
             echo "You might be asked for your password."
             echo "You can avoid this by adding your user to the 'docker' group,"
             echo "but be aware of the security implications."
             echo "See https://docs.docker.com/install/linux/linux-postinstall/"
             echo ""
-            sudo docker run --rm -it -v "$(pwd):/antora:z" "${image}" "${cmd}"
+            sudo docker run --rm -it -v "$(pwd):/antora:z" "${image}" ${cmd}
         fi
     else
         echo ""