getdeps: try harder to avoid ccache on sandcastle

Summary:
A small number of hosts are impacted by some local misconfiguration like this:

```
ccache /usr/bin/c++  -DHAVE_PCLMUL -DHAVE_SSE42 -DOS_LINUX -DROCKSDB_FALLOCATE_PRESENT -DROCKSDB_LIB_IO_POSIX -DROCKSDB_MALLOC_USABLE_SIZE -DROCKSDB_NO_DYNAMIC_EXTENSION -DROCKSDB_PLATFORM_POSIX -DROCKSDB_PTHREAD_ADAPTIVE_MUTEX -DROCKSDB_RANGESYNC_PRESENT -DROCKSDB_SCHED_GETCPU_PRESENT -DROCKSDB_SUPPORT_THREAD_LOCAL -DSNAPPY -I/var/sandcastle/temp/fbcode_builder_getdeps/extracted/rocksdb-v6.8.1.tar.gz/rocksdb-6.8.1 -I/var/sandcastle/temp/fbcode_builder_getdeps/extracted/rocksdb-v6.8.1.tar.gz/rocksdb-6.8.1/include -I/var/sandcastle/temp/fbcode_builder_getdeps/extracted/rocksdb-v6.8.1.tar.gz/rocksdb-6.8.1/third-party/folly -isystem /var/sandcastle/temp/fbcode_builder_getdeps/extracted/rocksdb-v6.8.1.tar.gz/rocksdb-6.8.1/third-party/gtest-1.8.1/fused-src -isystem /var/sandcastle/temp/fbcode_builder_getdeps/installed/snappy-8kzrVQJKWlYwtUKTyQTv0R3_bltgjXecrUEMwSpi-vI/include -W -Wextra -Wall -Wsign-compare -Wshadow -Wno-unused-parameter -Wno-unused-variable -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers -Wno-strict-aliasing -std=c++11 -fno-omit-frame-pointer -momit-leaf-frame-pointer -fno-builtin-memcmp -O2 -g -DNDEBUG -MD -MT CMakeFiles/rocksdb.dir/cache/lru_cache.cc.o -MF CMakeFiles/rocksdb.dir/cache/lru_cache.cc.o.d -o CMakeFiles/rocksdb.dir/cache/lru_cache.cc.o -c /var/sandcastle/temp/fbcode_builder_getdeps/extracted/rocksdb-v6.8.1.tar.gz/rocksdb-6.8.1/cache/lru_cache.cc
ccache: error: /home/facebook/.ccache/ccache.conf: No such file or directory
```

let's try to avoid it!

Reviewed By: simpkins

Differential Revision: D21460408

fbshipit-source-id: 95b1fe019ff36eb8acb8476af997c220a7947436

build/fbcode_builder/getdeps/builder.py

+#!/usr/bin/env python3
 # Copyright (c) Facebook, Inc. and its affiliates.
 #
 # This source code is licensed under the MIT license found in the
@@ -436,6 +437,12 @@ if __name__ == "__main__":
             # rocksdb does its own probing for ccache.
             # Ensure that it is disabled on sandcastle
             env["CCACHE_DISABLE"] = "1"
+            # Some sandcastle hosts have broken ccache related dirs, and
+            # even though we've asked for it to be disabled ccache is
+            # still invoked by rocksdb's cmake.
+            # Redirect its config directory to somewhere that is guaranteed
+            # fresh to us, and that won't have any ccache data inside.
+            env["CCACHE_DIR"] = f"{self.build_opts.scratch_dir}/ccache"
 
         if "GITHUB_ACTIONS" in os.environ and self.build_opts.is_windows():
             # GitHub actions: the host has both gcc and msvc installed, and