Use CIRCLRCI environment variable

When `__FILE__` macro is in code, this produces different paths in
different environments, making the builds not reproducible. We use
`-ffile-prefix-map` to avoid this problem but only when
`deterministic_paths` is True:
https://github.com/emscripten-core/emscripten/blob/d5c419f8aa428bc3d1d071d23227dd13a00b463c/tools/system_libs.py#L495-L501
which is set to True when embuilder is used in non-`USE_NINJA` mode:
https://github.com/emscripten-core/emscripten/blob/1171adaa21048ba8dc82169b9d79952e8754b201/embuilder.py#L296
The rationale for this seems to be that we sometimes want correct
absolute paths when we debug and test programs locally.

But CircleCI runs several code size tests and if the those tests contain
objects with `__FILE__` macro, the code sizes in those tests can be
different in different CircleCI bots.

This uses the environment variable `CIRCLECI`, which is True when we are
in CircleCI bots, to decide whether we need to set
`deterministic_paths`.

This was prompted due to a recent change in libc++abi:
https://github.com/llvm/llvm-project/blob/aadaa00de76ed0c4987b97450dd638f63a385bed/libcxxabi/src/abort_message.h#L22
This `__FILE__` ends up being used from
`other.test_minimal_runtime_code_size_hello_embind`, making the test's
result different in different bots. But this can happen in other places
and it is generally better to produce reproducible builds in CirclrCI
bots anyway.

Author: aheejin

tools/system_libs.py

@@ -41,6 +41,7 @@
 # Setting EMCC_USE_NINJA=2 means that ninja will automatically be run for each library needed at
 # link time.
 USE_NINJA = int(os.environ.get('EMCC_USE_NINJA', '0'))
+CIRCLECI = bool(os.environ.get('CIRCLECI', False))
 
 
 def files_in_path(path, filenames):
@@ -438,7 +439,7 @@ def get_link_flag(self):
 
     This will trigger a build if this library is not in the cache.
     """
-    fullpath = self.build()
+    fullpath = self.build(CIRCLECI)
     # For non-libraries (e.g. crt1.o) we pass the entire path to the linker
     if self.get_ext() != '.a':
       return fullpath