pick target during Xargo installation

ci/docker/thumbv7em-none-eabi/Dockerfile

@@ -3,7 +3,7 @@ RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev
 RUN curl -sf "https://raw.githubusercontent.com/japaric/rust-everywhere/master/install.sh" | \
-    bash -s -- --at /usr/bin --from japaric/xargo --tag v0.1.10
+    bash -s -- --at /usr/bin --from japaric/xargo --for x86_64-unknown-linux-gnu --tag v0.1.10
 ENV AR_thumbv7em_none_eabi=arm-none-eabi-ar \
     CARGO_TARGET_THUMBV7EM_NONE_EABI_LINKER=arm-none-eabi-gcc \
     CC_thumbv7em_none_eabi=arm-none-eabi-gcc \

ci/docker/thumbv7em-none-eabihf/Dockerfile

@@ -3,7 +3,7 @@ RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev
 RUN curl -sf "https://raw.githubusercontent.com/japaric/rust-everywhere/master/install.sh" | \
-    bash -s -- --at /usr/bin --from japaric/xargo --tag v0.1.10
+    bash -s -- --at /usr/bin --from japaric/xargo --for x86_64-unknown-linux-gnu --tag v0.1.10
 ENV AR_thumbv7em_none_eabihf=arm-none-eabi-ar \
     CARGO_TARGET_THUMBV7EM_NONE_EABIHF_LINKER=arm-none-eabi-gcc \
     CC_thumbv7em_none_eabihf=arm-none-eabi-gcc \

ci/docker/thumbv7m-none-eabi/Dockerfile

@@ -3,7 +3,7 @@ RUN apt-get update
 RUN apt-get install -y --no-install-recommends \
     ca-certificates curl gcc gcc-arm-none-eabi libc6-dev libcurl4-openssl-dev libssh2-1 libnewlib-dev
 RUN curl -sf "https://raw.githubusercontent.com/japaric/rust-everywhere/master/install.sh" | \
-    bash -s -- --at /usr/bin --from japaric/xargo --tag v0.1.10
+    bash -s -- --at /usr/bin --from japaric/xargo --for x86_64-unknown-linux-gnu --tag v0.1.10
 ENV AR_thumbv7m_none_eabi=arm-none-eabi-ar \
     CARGO_TARGET_THUMBV7M_NONE_EABI_LINKER=arm-none-eabi-gcc \
     CC_thumbv7m_none_eabi=arm-none-eabi-gcc \