Use shtk from HEAD during GitHub testing

We need the future shtk 1.8 but it hasn't been released yet, so pull
shtk from Git for now.  I'll revert back to 1.8 once a new sysbuild
release is ready to be cut.

.github/workflows/install-shtk.sh

@@ -28,7 +28,7 @@
 
 set -eux
 
-readonly VERSION=1.7
+readonly VERSION=HEAD
 
 main() {
     local prefix="${1}"; shift
@@ -39,9 +39,14 @@ main() {
 
     cd "${tmpdir}"
 
-    curl -LO "https://github.com/jmmv/shtk/releases/download/shtk-${VERSION}/shtk-${VERSION}.tar.gz"
-    tar xzvf "shtk-${VERSION}.tar.gz"
-    cd "shtk-${VERSION}"
+    if [ "${VERSION}" = HEAD ]; then
+        git clone https://github.com/jmmv/shtk.git
+        cd shtk
+    else
+        curl -LO "https://github.com/jmmv/shtk/releases/download/shtk-${VERSION}/shtk-${VERSION}.tar.gz"
+        tar xzvf "shtk-${VERSION}.tar.gz"
+        cd "shtk-${VERSION}"
+    fi
     ./configure --prefix="${prefix}"
     make
     sudo make install