docker/windows: Update msys2 installer to 2021-02-15 version.

This fixes the PGP key installation issue.

Part-of: <!396>
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Status Job ID Name Coverage
  Build Docker
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alpine amd64 manifest builder docker

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android docker

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cerbero fedora amd64 docker

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fedora amd64 docker

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gst-indent amd64 docker

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test manifest amd64 docker

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  Preparation
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gst indent

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manual
manifest
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test manifest

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  Build
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gstreamer
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gst-ios-14.3
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gstreamer
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gstreamer
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gst-macos-11.1
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docker windows gstreamer-windows 1809
build cerbero msvc x86_64
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build clang fedora x86_64
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build gst-omx tizonia fedora x86_64
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build gst-omx zynq fedora x86_64
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docker windows 1809 allowed to fail manual
build msys2
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build nodebug fedora x86_64
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build static fedora x86_64
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build static nodebug fedora x86_64
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docker windows 1809
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docker windows 1809
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  Test
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gstreamer
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  Integrate
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cross-android universal examples
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gst-ios-14.3
cross-ios universal examples
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documentation
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gstreamer-full
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gstreamer-full-minimal