Fall back to Go 1.26 with a warning when the version file is missing

[sverdlov93]

Problem

Since #28 this action defaults to go-version-file: go.mod instead of a concrete go-version. actions/setup-go treats a missing version file as fatal, so callers that do not pass go-version and do not have the file in the workspace die on the first step.

Change

When go-version is not set and go-version-file does not exist, install Go 1.26 and warn. Nothing else changes: an existing version file is still read, and an explicit go-version still wins.

1.26 is the previous supported major (current is 1.27). A census of the 19 public jfrog/* callers shows flagship CLIs already on 1.26.x; older modules (1.22–1.25) never hit this path because they have a go.mod.

Warning: The configured go-version-file does not exist in the workspace, so Go 1.26 is installed instead.
Run actions/checkout before this action, or pass go-version explicitly to silence this warning.

Resolution table

go-version	version file	setup-go receives	Warning
1.24	any	go-version=1.24	no
—	exists	go-version-file=<path>	no
—	missing	go-version=1.26	yes