ci: set umask 022 to prevent group-writable cache dirs (sr.ht default umask creates 0775 dirs)

ober

7dbf630aea14a5807c25999a95535cff86b2d3b2

diff --git a/.build.yml b/.build.yml
index e1b00c1..cd65c16 100644
--- a/.build.yml
+++ b/.build.yml
@@ -35,6 +35,7 @@ tasks:
   - build-jerboa-tools: |
       # Build the current multicall tool so CI exercises the same
       # jerboa/jerbuild/jpkg image users install.
+      umask 022
       cd jerboa
       curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
         sh -s -- -y --profile minimal --default-toolchain 1.94.1