Makefile: update shellcheck source

The old Shellcheck download location is no longer valid.

xref: koalaman/shellcheck#1871

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

Makefile

@@ -215,7 +215,7 @@ $(GOLANGCILINT):
  | sh -s -- -b $(FIRST_GOPATH)/bin $(GOLANGCILINT_VERSION)
 
 $(SHELLCHECK): $(BIN_DIR)
- curl -sNL "https://storage.googleapis.com/shellcheck/shellcheck-stable.$(OS).$(ARCH).tar.xz" | tar --strip-components=1 -xJf - -C $(BIN_DIR)
+ curl -sNL "https://github.com/koalaman/shellcheck/releases/download/stable/shellcheck-stable.$(OS).$(ARCH).tar.xz" | tar --strip-components=1 -xJf - -C $(BIN_DIR)
 
 $(GENERATE_TLS_CERT): | vendor $(BIN_DIR)
  # A thin wrapper around github.com/cloudflare/cfssl