ci: build the az tdx attester module in az podvm

Only the SEV-SNP attester module was included in the build so far, this
sets the az-tdx-vtpm feature on the attestation-agent build.

Signed-off-by: Magnus Kulke <magnuskulke@microsoft.com>

.github/workflows/azure-podvm-image-build.yml

@@ -110,7 +110,9 @@ jobs:
         rm -r "${GOPATH}/bin/yq"
 
     - name: Build binaries
-      run: make binaries LIBC=gnu AA_KBC=cc_kbc_az_snp_vtpm
+      run: make binaries \
+        AA_KBC="cc_kbc_az_snp_vtpm,cc_kbc_az_tdx_vtpm" \
+        LIBC=gnu
 
     - uses: azure/login@v1
       name: 'Az CLI login'