Cache swift-format (#1887)

Motivation:

The CI takes ~7 minutes to run, the longest job is the formatting check
which spends most of its time compiling swift-format.

Modifications:

- Cache the swift-format build directory

Result:

Faster CI

scripts/license-check.sh

@@ -102,6 +102,10 @@ check_copyright_headers() {
     ! -name '*.grpc.swift' \
     ! -name 'LinuxMain.swift' \
     ! -name 'XCTestManifests.swift' \
+    ! -path './FuzzTesting/.build/*' \
+    ! -path './Performance/QPSBenchmark/.build/*' \
+    ! -path './Performance/Benchmarks/.build/*' \
+    ! -path './scripts/.swift-format-source/*' \
     ! -path './.build/*')
 }