Do apt-get update before apt-get install

Analog change to an update necessary in -ukbiobank that makes sense in
general.

Fixes datalad/datalad-extension-template#13

.appveyor.yml

@@ -153,7 +153,7 @@ install:
       )
   - sh: python -m pip install datalad-installer${DATALAD_INSTALLER_VERSION:-}
   # Missing system software
-  - sh: "[ -n \"$INSTALL_SYSPKGS\" ] && ( [ \"x${APPVEYOR_BUILD_WORKER_IMAGE}\" = \"xmacOS\" ] && brew install -q ${INSTALL_SYSPKGS} || sudo apt-get install --no-install-recommends -y ${INSTALL_SYSPKGS} ) || true"
+  - sh: "[ -n \"$INSTALL_SYSPKGS\" ] && ( [ \"x${APPVEYOR_BUILD_WORKER_IMAGE}\" = \"xmacOS\" ] && brew install -q ${INSTALL_SYSPKGS} || { sudo apt-get update -y && sudo apt-get install --no-install-recommends -y ${INSTALL_SYSPKGS}; } ) || true"
   # Install git-annex on windows, otherwise INSTALL_SYSPKGS can be used
   # deploy git-annex, if desired
   - cmd: IF DEFINED INSTALL_GITANNEX datalad-installer --sudo ok %INSTALL_GITANNEX%