[Fuchsia] Roll back Fuchsia SDK to 0.20200604.2.1

Fuchsia x64 bots started flaking on 6/04 due to a pkgfs issue.
https://fuchsia-review.googlesource.com/c/fuchsia/+/396849 was landed in
fuchsia to workaround that issue. That fix is included in
0.20200609.1.1 SDK. After that roll pkgfs stopped failing, but tests
were still flaking with about the same frequency. The flake may be related
to femu issue introduced about the same time, see fxbug.dev/53770, but the
root cause is unknown.

Temporarily revert to 0.20200604.2.1 which is expected to be stable.

TBR=wez@chromium.org

Bug: 1092054
Change-Id: Ib14a67f0efe9cbf01962d703cb9b1870298534c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239038Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776877}

build/fuchsia/linux.sdk.sha1

-0.20200609.2.1
+0.20200604.2.1

build/fuchsia/mac.sdk.sha1

-0.20200609.2.1
+0.20200604.2.1