[roll third_party/pigweed] pw_emu: Add support for substitutions in config entries

For now only $pw_env{} is supported. Note that the pw_env_setup also
performs this substitution, but this does not work when using
pw_emu configuration fragments.

Ideally this should be done in a generic fashion for all Pigweed
modules (b/309161337) but this is out of scope for this CL which aims
at allowing downstream projects to perform substitution for emulation
target fragments (e.g. for using a project specific qemu binary
relative to the project root).

This CL also adds unit tests for pw_emu.core.Config.

Original-Bug: 308793747
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/179150

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 42440ce40b2db3b..05b7e67802a614b
Roller-URL: https://ci.chromium.org/b/8765061394993642209
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I87e1a922f027af220c44623533db1be33b5d73ff
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/179274
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 42440ce..05b7e67 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 42440ce40b2db3b316902cef48c96830819c716c
+Subproject commit 05b7e67802a614b788d437e1d734cc7bf4b7d30d