pkg/epoch: replace some fmt.Sprintfs with strconv

Teeny-tiny optimizations:

    BenchmarkSprintf-10       37735996    32.31  ns/op  0 B/op  0 allocs/op
    BenchmarkItoa-10         591945836     2.031 ns/op  0 B/op  0 allocs/op
    BenchmarkFormatUint-10   593701444     2.014 ns/op  0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/epoch/epoch.go

@@ -60,10 +60,10 @@ func SourceDateEpochOrNow() time.Time {
 
 // SetSourceDateEpoch sets the SOURCE_DATE_EPOCH env var.
 func SetSourceDateEpoch(tm time.Time) {
-	os.Setenv(SourceDateEpochEnv, fmt.Sprintf("%d", tm.Unix()))
+	_ = os.Setenv(SourceDateEpochEnv, strconv.Itoa(int(tm.Unix())))
 }
 
 // UnsetSourceDateEpoch unsets the SOURCE_DATE_EPOCH env var.
 func UnsetSourceDateEpoch() {
-	os.Unsetenv(SourceDateEpochEnv)
+	_ = os.Unsetenv(SourceDateEpochEnv)
 }