CI: Point exprtk to release branch

Author: ArashPartow

Diff for: docker/alpine-normal/Dockerfile

@@ -232,7 +232,7 @@ RUN --mount=type=cache,id=alpine-normal-spatialite,target=$HOME/.cache \
     fi
 
 # Download exprtk
-RUN curl -LO -fsS https://github.com/ArashPartow/exprtk/raw/refs/heads/master/exprtk.hpp \
+RUN curl -LO -fsS https://github.com/ArashPartow/exprtk/raw/refs/heads/release/exprtk.hpp \
     && mv exprtk.hpp /usr/include
 
 # Build GDAL

Diff for: docker/ubuntu-full/Dockerfile

@@ -495,7 +495,7 @@ RUN --mount=type=cache,id=ubuntu-full-proj,target=$HOME/.cache \
     && /buildscripts/bh-proj.sh
 
 # Download exprtk
-RUN curl -LO -fsS https://github.com/ArashPartow/exprtk/raw/refs/heads/master/exprtk.hpp \
+RUN curl -LO -fsS https://github.com/ArashPartow/exprtk/raw/refs/heads/release/exprtk.hpp \
     && mv exprtk.hpp /usr/include
 
 # Build GDAL