GitHub Actions: enable toolchain management on Fedora

testdata/ci-fedora.Dockerfile

@@ -3,3 +3,11 @@ FROM fedora
 
 # Install rsync (for running tests).
 RUN dnf -y update && dnf -y install rsync openssh-clients go && dnf clean all
+
+# Enable toolchain management (and the module proxy, which is a requirement) so
+# that Go 1.23 (from Fedora) will use Go 1.24 for gokrazy/rsync (or whichever
+# version we specify as language/toolchain version in our go.mod).
+RUN go env -w \
+    GOPROXY=https://proxy.golang.org,direct \
+    GOSUMDB=sum.golang.org \
+    GOTOOLCHAIN=auto