Fix string(int) issues for Go 1.15 compatibility

[jalseth]

go vet in Golang 1.15 does not allow type casting from int to string. More info in golang/go#32479 and golang/go#3939.

Example output:

$ make test
go vet ./...
# github.com/kolide/fleet/server/datastore
server/datastore/datastore_labels_test.go:22:29: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
server/datastore/datastore_labels_test.go:22:40: conversion from int to string yields a string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)
make: *** [lint-go] Error 2

[zwass]

Thank you!