externalsrc: place copy of git index into /tmp and do not use copyfile2

Using shutil.copy2() to copy .git/index to a temporary file tries to
copy SELinux attributes which might fail for confined users in SELinux
environments.

E.g. our builders are running in docker containers and modification of
sources (inclusive updated of .git/index) is done outside.  Trying to
copy .git/index fails with

| $ python3 -c 'import shutil; shutil.copy2("index", "a")'
| ...
| PermissionError: [Errno 13] Permission denied: 'a'

and an AVC like

| denied  { relabelto } for  pid=18043 comm="python3" name="a" dev="dm-29" ino=1067553 scontext=system_u:system_r:container_t:s0:c39,c558 tcontext=unconfined_u:object_r:build_file_t:s0 tclass=file permissive=0

is created.  This can not be solved by adapting the SELinux policy because
this is a very deep constraint violation:

| constrain file { create relabelfrom relabelto } ((u1 == u2 -Fail-)  or (t1 == can_change_object_identity -Fail-) ); Constraint DENIED
|
| Possible cause is the source user (system_u) and target user (unconfined_u) are different.

I do not see much sense in using 'shutil.copy2()' here; 'shutil.copyfile()'
seems to be a better choice (target file is created in a secure way by
tempfile.NamedTemporaryFile()).

By placing the tempfile into /tmp we avoid potential problems related to
git's 'core.sharedRepository'.  As a (positive) side effect, the source
tree will not be modified anymore (at least by this part of code) which
prevented to mount it read-only from somewhere else.

(From OE-Core rev: 3c3c8ecc61dfed68987750d79b5482ab2f6fa02f)

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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