dd-agent: fix compatibility issue with iostat (#41035)

Applies a patch to the dd-agent derivation that fixes a compatibility
issue with the current version of iostat, which no longer contains a
colon after its table headers.

This patch is applied in order for the fix to be backportable to
existing stable releases. A final "proper" fix will be an upgrade to a
newer version of dd-agent, but that requires several other changes.

This fixes #40103.