specter: Fix build in emulated ARM32v7 envirnoments

See pyca/cryptography#5771,
rust-lang/cargo#8719

specter/install

@@ -21,6 +21,15 @@ HOME=/root pip3 install --upgrade pip==21.1.3
 # 'setup.py' imports it.
 s6-setuidgid specter pip3 install --user babel==2.9.1
 
+# ARM7 doesn't have pre-built wheels for the python 'cryptography' lib, whuch
+# normally requires Rust, which does not work properly in an emulated ARM 7
+# envirnoment. The flag below is a temporary workaround which is expected to
+# become unavailable in the next minor cryptography release (v3.5)
+# https://github.com/pyca/cryptography/issues/5771
+# https://github.com/rust-lang/cargo/issues/8719
+[ $(dpkg --print-architecture) == "armhf" ] && \
+  export CRYPTOGRAPHY_DONT_BUILD_RUST=1
+
 # Install Specter
 wget -qO /tmp/specterd.tar.gz https://github.com/cryptoadvance/specter-desktop/archive/v$SPECTER_VERSION.tar.gz
 echo "$SPECTER_SHA256 /tmp/specterd.tar.gz" | sha256sum -c -