tools: mkimage-*: disable content trust

Signing is not setup for these repositories. It seems they haven't been updated
since before trust was added since their previous Makefile metadata (which was
translated in build.yml) did not contain `NOTRUST=1`.

Signed-off-by: Ian Campbell <ijc@docker.com>

tools/mkimage-dynamic-vhd/build.yml

@@ -1 +1,2 @@
 image: mkimage-dynamic-vhd
+disable-content-trust: true

tools/mkimage-gcp/build.yml

@@ -1 +1,2 @@
 image: mkimage-gcp
+disable-content-trust: true

tools/mkimage-iso-bios/build.yml

@@ -1,3 +1,4 @@
 image: mkimage-iso-bios
+disable-content-trust: true
 arches:
   - amd64

tools/mkimage-iso-efi/build.yml

@@ -1,2 +1,3 @@
 image: mkimage-iso-efi
+disable-content-trust: true
 network: true

tools/mkimage-vhd/build.yml

@@ -1 +1,2 @@
 image: mkimage-vhd
+disable-content-trust: true

tools/mkimage-vmdk/build.yml

@@ -1 +1,2 @@
 image: mkimage-vmdk
+disable-content-trust: true