Set ZERO_AR_DATE=1 to eliminate timestamp from .a outputs

Summary:
By default, the archive header produced by `ar q` embeds the current unix timestamp.

The GNU archiver provides `ar qD` to make a deterministic archive, which we already use in Buck if supported.

https://www.internalfb.com/code/fbsource/[b6ae1a41be80e4b62a2595c69f0502b7f71b3103]/fbcode/buck2/prelude/cxx/archive.bzl?lines=44-46

The darwin archiver does not support `qD`.

```lang=text
/bin/ar: illegal option -- D
usage:  ar -d [-TLsv] archive file ...
ar -m [-TLsv] archive file ...
ar -m [-abiTLsv] position archive file ...
ar -p [-TLsv] archive [file ...]
ar -q [-cTLsv] archive file ...
ar -r [-cuTLsv] archive file ...
ar -r [-abciuTLsv] position archive file ...
ar -t [-TLsv] archive [file ...]
ar -x [-ouTLsv] archive [file ...]
```

It instead supports the same behavior through an environment variable `ZERO_AR_DATE`.

Reviewed By: d16r

Differential Revision: D60680127

fbshipit-source-id: 628e12b5ea4b209ba92582722fe49bc8e24ad5c3

cxx/archive.bzl

@@ -78,10 +78,24 @@ def _archive(ctx: AnalysisContext, name: str, args: cmd_args, thin: bool, prefer
     else:
         command.add(args)
 
+    # By default, the archive header produced by `ar q` embeds the current unix
+    # timestamp. With the GNU archiver we use `ar qD` (above in _archive_flags)
+    # to make it produce a deterministic archive by zeroing the timestamp, but
+    # other archivers do not support such a flag. Some implementations, notably
+    # Xcode's, instead support zeroing the timestamp by way of an environment
+    # variable.
+    env = {"ZERO_AR_DATE": "1"}
+
     category = "archive"
     if thin:
         category = "archive_thin"
-    ctx.actions.run(command, category = category, identifier = name, prefer_local = prefer_local)
+    ctx.actions.run(
+        command,
+        category = category,
+        identifier = name,
+        env = env,
+        prefer_local = prefer_local,
+    )
     return archive_output
 
 def _archive_locally(ctx: AnalysisContext, linker_info: LinkerInfo) -> bool: