Bump MCUBOOT_REFSPEC to 81d19f017f18f1bc6bea906d96fd45fba3672064

Align with new version used in TF-M:
* 81d19f0 bootutil: Add cmake build file
 In order to allow other projects to include the bootutil files more
 easily. Allows renaming and moving of bootutil files without breaking
 external projects' file lists (if they include this cmake file instead
 of directly listing the files they use). Prevents an issue where
 moving/renaming bootutil files breaks the FIH CI test.

Signed-off-by: Raef Coles <raef.coles@arm.com>
Change-Id: Ib510a1fe2480f522c0cc3b20213722fc25a73017
diff --git a/tf-m-build-docs-nightly.yaml b/tf-m-build-docs-nightly.yaml
index 6fd35be..93046dc 100644
--- a/tf-m-build-docs-nightly.yaml
+++ b/tf-m-build-docs-nightly.yaml
@@ -59,7 +59,7 @@
           default: 'master'
       - string:
           name: 'MCUBOOT_REFSPEC'
-          default: 'refs/tags/v1.7.0'
+          default: '81d19f017f18f1bc6bea906d96fd45fba3672064'
           description: |
             Specify which MCUboot version to use
             refs/heads/master    : Pick up latest master