github: Set the versions to build in scheduled builds via the GHA "env" field

For non scheduled builds, those env variables are just left empty.

This is much neater than using the "versions" file, and uses much
fewer lines of code.

We still probably want to bundle a file labelling what versions
it contains in the built packages though; give that file a
.txt suffix.

.github/workflows/store-version.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+OUT="$1"
+
+if [ -n "$LLVM_VERSION" ]; then
+    echo "Nightly $(TZ=UTC date +%Y-%m-%d)" >> $OUT
+    for i in LLVM_VERSION MINGW_W64_VERSION PYTHON_VERSION_MINGW; do
+        eval val=\"\$$i\"
+        if [ -n "$val" ]; then
+            echo "$i=$val" >> $OUT
+        fi
+    done
+fi