fix conversion for int/uint in ReplaceVariables

[jaypipes]

There was an unsafe type conversion in the type switch statement in cotnext.ReplaceVariables() that caused a panic that looked like this:

panic: interface conversion: interface {} is int64, not int
goroutine 147 [running]:
github.com/gdt-dev/core/context.ReplaceVariables({0x105cbd6c8?, 0x140007a5030?}, {0x14000042480, 0xc})
/Users/abdul/go/pkg/mod/github.com/gdt-dev/core@v1.10.4/context/getter.go:132 +0x31c
github.com/gdt-dev/kube.(*Action).get(0x140000b2a20, {0x105cbd6c8, 0x140007a5030}, 0x14000483530, {0x1400027aab8, 0x5}, 0x14000053d00)
/Users/abdul/go/pkg/mod/github.com/gdt-dev/kube@v1.10.3/action.go:126 +0x60
github.com/gdt-dev/kube.(*Action).Do(0x105cbd6c8?, {0x105cbd6c8?, 0x140007a5030?}, 0x1400027aab8?, {0x1400027aab8?, 0x1046819ec?}, 0x14000567cc8?)
/Users/abdul/go/pkg/mod/github.com/gdt-dev/kube@v1.10.3/action.go:104 +0xb8
github.com/gdt-dev/kube.(*Spec).Eval(0x140000c2d80, {0x105cbd6c8, 0x140007a5030})
/Users/abdul/go/pkg/mod/github.com/gdt-dev/kube@v1.10.3/eval.go:37 +0x1e8
github.com/gdt-dev/core/scenario.(*Scenario).execSpec(0x0?, {0x105cbd6c8, 0x140007a5030}, 0x140007a4fc0, 0x140002406c0, 0x0?, {0x105cbf320, 0x140000c2d80})
/Users/abdul/go/pkg/mod/github.com/gdt-dev/core@v1.10.4/scenario/run.go:369 +0x3ac1

This patch fixes that unsafe type conversion and uses the recommended strconv.FormatInt and strconv.FormatUint functions for safe conversion of int64 and uint64 to string.

Issue gdt-dev/gdt#72