tar: Do not use String(format:) in octal6/11

String(format:) sometimes returns an empty string even if the number
being formatted can be represented in an octal string of the requested
length.  This may be a thread-safety problem and has only been seen
when running the tests.

    swiftlang/swift-corelibs-foundation#5152

This commit changes octal6/11 to use String(value, radix:) and
handle padding directly.   This has not failed during hundreds of
test runs.

Benchmarking the old and new implementations of octal6() shows that
the new version also is about twice as fast as the old one and makes
no allocations, whereas the old version made 7 allocations.

Sources/Tar/tar.swift

@@ -72,7 +72,8 @@ extension [UInt8] {
 func octal6(_ value: Int) -> String {
     precondition(value >= 0)
     precondition(value < 0o777777)
-    return String(format: "%06o", value)
+    let str = String(value, radix: 8)
+    return String(repeating: "0", count: 6 - str.count).appending(str)
 }
 
 /// Serializes an integer to a 11 character octal representation.
@@ -81,7 +82,8 @@ func octal6(_ value: Int) -> String {
 func octal11(_ value: Int) -> String {
     precondition(value >= 0)
     precondition(value < 0o777_7777_7777)
-    return String(format: "%011o", value)
+    let str = String(value, radix: 8)
+    return String(repeating: "0", count: 11 - str.count).appending(str)
 }
 
 // These ranges define the offsets of the standard fields in a Tar header.