ci(e2e): bump chainsaw to v0.2.15 for Go 1.26

chainsaw v0.2.13 fails to build under Go 1.26: its internal TestDeps no longer
satisfies testing.testDeps (Go 1.24+ added the ModulePath method), breaking the
"Prepare e2e" step's `make chainsaw`. v0.2.15 implements it and builds cleanly
under Go 1.26 (verified locally).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells

Makefile

@@ -253,7 +253,7 @@ CRDOC_VERSION ?= v0.6.4
 KIND_VERSION ?= v0.27.0
 
 # renovate: datasource=go depName=github.com/kyverno/chainsaw
-CHAINSAW_VERSION ?= v0.2.13
+CHAINSAW_VERSION ?= v0.2.15
 
 # renovate: datasource=go depName=github.com/cert-manager/cmctl/v2
 CMCTL_VERSION ?= v2.1.1