If anyone is interested I came across this same subject on a blog post and discussion on HN today:
On 2018-04-02 15:03, Guido van Rossum wrote:
IIRC Algol-68 (the lesser-known, more complicated version) used 'int x = 0;' to declare a constant and 'int x := 0;' to declare a variable. And there was a lot more to it; see https://en.wikipedia.org/wiki/ALGOL_68#mode:_Declarations. I'm guessing Go reversed this because they want '=' to be the common assignment (whereas in Algol-68 the common assignment was ':=').