proto: tests fail to build on Go 1.15 due to new vet check

[saser]

What version of protobuf and what language are you using?
Version: v1.4.2.

$ go version
go version go1.15.2 linux/amd64

What did you do?
Go 1.15 contains a new vet check that warns for conversion of the form string(x) where x is an integer type that is not rune or byte. I initially noticed the build failure when I ran go test all in a project that uses Go 1.15 and depends on this module.

To reproduce:

$ cd $(mktemp -d)

$ go mod init test
go: creating new go.mod: module test

$ go get github.com/golang/[email protected]
go: downloading github.com/golang/protobuf v1.4.2

$ go test github.com/golang/protobuf/proto
go: downloading google.golang.org/protobuf v1.23.0
go: downloading github.com/google/go-cmp v0.4.0
# github.com/golang/protobuf/proto
[redacted $GOPATH]/pkg/mod/github.com/golang/[email protected]/proto/text_decode.go:768:10: conversion from uint64 to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
FAIL    github.com/golang/protobuf/proto [build failed]
FAIL

What did you expect to see?
Passing tests.

What did you see instead?
A build failure (see details above).

Anything else we should know about your project / environment?
N/A