test: Set the permissions explicitly for the copied ChangeLog.old file

The source ChangeLog.old file might have different permissions depending
on the umask during the source package unpack. Copy the file without
any flags, and then explicitly set known permissions on the result file.

This should fix the build on reproducible variants.

Fixes: commit 2c2f7066

src/at/deb-format.at

@@ -28,7 +28,8 @@ AT_DATA([pkg-deb-old-format/file-templ], [test
 AT_CHECK([
 # Initialize the template package
 mv pkg-deb-old-format pkg-old-templ
-cp -a $top_srcdir/ChangeLog.old pkg-old-templ/
+cp $top_srcdir/ChangeLog.old pkg-old-templ/
+chmod 0644 pkg-old-templ/ChangeLog.old
 chmod -R u+w pkg-old-templ
 find pkg-old-templ | xargs touch -t 197001010100.00
 dpkg-deb --deb-format=0.939000 --root-owner-group -Zgzip -b pkg-old-templ >/dev/null
@@ -188,7 +189,8 @@ AT_DATA([pkg-deb-format/file-templ], [test
 AT_CHECK([
 # Initialize the template package
 mv pkg-deb-format pkg-templ
-cp -a $top_srcdir/ChangeLog.old pkg-templ/
+cp $top_srcdir/ChangeLog.old pkg-templ/
+chmod 0644 pkg-templ/ChangeLog.old
 chmod -R u+w pkg-templ
 find pkg-templ | xargs touch -t 197001010100.00
 dpkg-deb --uniform-compression --root-owner-group -Znone -b pkg-templ >/dev/null