Protect package installation against interruption

If rsync is interrupted by the user while installing package files to their
final path, it might already have copied the .build-hash file. That would
cause aliBuild to assume that the package was fully installed on subsequent
runs, even if some parts are missing.

Protect against this by explicitly creating the marker file last, after rsync
has succeeded.

alibuild_helpers/build_template.sh

@@ -155,7 +155,6 @@ cat <<\EOF > "$INSTALLROOT/etc/profile.d/init.sh"
 EOF
 
 cd "$WORK_DIR/INSTALLROOT/$PKGHASH"
-echo "$PKGHASH" > "$INSTALLROOT/.build-hash"
 # Replace the .envrc to point to the final installation directory.
 cat << EOF > "$BUILDDIR/.envrc"
 # Source the build environment which was used for this package
@@ -293,6 +292,9 @@ if [[ $BUILD_FAMILY ]]; then
   ln -snf $PKGVERSION-$PKGREVISION $ARCHITECTURE/$PKGNAME/latest-$BUILD_FAMILY
 fi
 
+# When the package is definitely fully installed, install the file that marks
+# the package as successful.
+echo "$PKGHASH" > "$WORK_DIR/$PKGPATH/.build-hash"
 # Mark the build as successful with a placeholder. Allows running incremental
 # recipe in case the package is in development mode.
 echo "${DEVEL_HASH}${DEPS_HASH}" > "$BUILDDIR/.build_succeeded"