[TESTING] Update cy.yml to test with SCP-firmware PR812

This patch changes download_scp_firmware() in the ci github workflow
only to test github Pull Request PR812 in SCP-firmware.

This change is NOT to be merged.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>

.github/workflows/ci.yml

@@ -81,7 +81,7 @@ jobs:
 
           function _make() { make -j$(nproc) -s O=out $*; }
           function download_plug_and_trust() { mkdir -p $HOME/se050 && git clone --single-branch -b v0.4.2 https://github.com/foundriesio/plug-and-trust $HOME/se050/plug-and-trust || (rm -rf $HOME/se050 ; echo Nervermind); }
-          function download_scp_firmware() { git clone --single-branch https://github.com/ARM-software/SCP-firmware.git $HOME/scp-firmware || echo Nervermind; }
+          function download_scp_firmware() { git clone --single-branch https://github.com/ARM-software/SCP-firmware.git $HOME/scp-firmware && { cd $HOME/scp-firmware && git fetch origin pull/812/head && git checkout FETCH_HEAD; } || echo Nevermind; }
 
           ccache -s -v
           download_plug_and_trust