Set GOTOOLCHAIN in Makefile

This avoids issues when the container image used to run the tests
have a different go version than what we use.
For example, golanci-lint panics if it was built using
go1.25 but go1.26 is used to run it.

CAPI also does this.

Signed-off-by: Lennart Jern <lennart.jern@est.tech>

Author: lentzi90

Makefile

@@ -29,6 +29,10 @@ TRACE ?= 0
 # Go
 GO_VERSION ?= 1.25.8
 
+# Ensure correct toolchain is used
+GOTOOLCHAIN = go$(GO_VERSION)
+export GOTOOLCHAIN
+
 # Directories.
 ARTIFACTS ?= $(REPO_ROOT)/_artifacts
 TOOLS_DIR := hack/tools