[cipd] Update to 2.6.15.

This is to pick up 95f2bed7f4
which should help with cache invalidation woes (specifically should help
prevent evicting cache items which are just about to be used).

This has already been rolled out to swarming.

R=bryner, chanli, fancl

Bug: 1374490
Change-Id: I0377c2092fc59c36c49eed8a0960df6b34cb73df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4599527
Reviewed-by: Chan Li <chanli@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

cipd_client_version

@@ -1 +1 @@
-git_revision:d605bc42029140862091a8ef7630961ff9a87fc5
+git_revision:66a1c73609268be7b0cbfc0054856abd97f1eeef

cipd_client_version.digests

@@ -1,25 +1,25 @@
 # This file was generated by
 #
 #  cipd selfupdate-roll -version-file cipd_client_version \
-#      -version git_revision:d605bc42029140862091a8ef7630961ff9a87fc5
+#      -version git_revision:66a1c73609268be7b0cbfc0054856abd97f1eeef
 #
 # Do not modify manually. All changes will be overwritten.
 # Use 'cipd selfupdate-roll ...' to modify.
 
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+linux-ppc64le   sha256  9a3fb1b8108dc172e5f7dee3cd8b7ea7a86402e4e3c6129ee4e763efc5166529
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