Add retries to a number of downloads

library/stdarch/.travis.yml

@@ -113,7 +113,7 @@ matrix:
     - env: TARGET=x86_64-pc-windows-gnu STDSIMD_DISABLE_ASSERT_INSTR=1
     - env: TARGET=i686-pc-windows-gnu
 
-install: rustup target add $TARGET
+install: travis_retry rustup target add $TARGET
 script:
   - cargo generate-lockfile
   - ci/run-docker.sh $TARGET $FEATURES

library/stdarch/ci/android-install-ndk.sh

@@ -11,7 +11,7 @@
 
 set -ex
 
-curl -O \
+curl --retry 5 -O \
      https://dl.google.com/android/repository/android-ndk-r15b-linux-x86_64.zip
 unzip -q android-ndk-r15b-linux-x86_64.zip
 

library/stdarch/ci/android-install-sdk.sh

@@ -19,7 +19,7 @@ set -ex
 # which apparently magically accepts the licenses.
 
 mkdir sdk
-curl https://dl.google.com/android/repository/sdk-tools-linux-3859397.zip -O
+curl --retry 5 https://dl.google.com/android/repository/sdk-tools-linux-3859397.zip -O
 unzip -d sdk sdk-tools-linux-3859397.zip
 
 case "$1" in

library/stdarch/ci/android-sysimage.sh

@@ -24,7 +24,7 @@ main() {
     apt-get install --no-install-recommends e2tools
 
     pushd "$td"
-    curl -O "${URL}/${name}"
+    curl --retry 5 -O "${URL}/${name}"
     unzip -q "${name}"
 
     local system