Download the GCC artifacts from the HTTP server instead of FTP server.

The former seems much more stable, in case the cache becomes invalidated.

src/ci/docker/dist-i686-linux/build-gcc.sh

@@ -17,6 +17,23 @@ GCC=4.8.5
 
 curl https://ftp.gnu.org/gnu/gcc/gcc-$GCC/gcc-$GCC.tar.bz2 | tar xjf -
 cd gcc-$GCC
+
+# FIXME(#49246): Remove the `sed` below.
+#
+# On 2018 March 21st, two Travis builders' cache for Docker are suddenly invalidated. Normally this
+# is fine, because we just need to rebuild the Docker image. However, it reveals a network issue:
+# downloading from `ftp://gcc.gnu.org/` from Travis (using passive mode) often leads to "Connection
+# timed out" error, and even when the download completed, the file is usually corrupted. This causes
+# nothing to be landed that day.
+#
+# We observed that the `gcc-4.8.5.tar.bz2` above can be downloaded successfully, so as a stability
+# improvement we try to download from the HTTPS mirror instead. Turns out this uncovered the third
+# bug: the host `gcc.gnu.org` and `cygwin.com` share the same IP, and the TLS certificate of the
+# latter host is presented to `wget`! Therefore, we choose to download from the insecure HTTP server
+# instead here.
+#
+sed -i'' 's|ftp://gcc\.gnu\.org/|http://gcc.gnu.org/|g' ./contrib/download_prerequisites
+
 ./contrib/download_prerequisites
 mkdir ../gcc-build
 cd ../gcc-build

src/ci/docker/dist-x86_64-linux/build-gcc.sh

@@ -17,6 +17,23 @@ GCC=4.8.5
 
 curl https://ftp.gnu.org/gnu/gcc/gcc-$GCC/gcc-$GCC.tar.bz2 | tar xjf -
 cd gcc-$GCC
+
+# FIXME(#49246): Remove the `sed` below.
+#
+# On 2018 March 21st, two Travis builders' cache for Docker are suddenly invalidated. Normally this
+# is fine, because we just need to rebuild the Docker image. However, it reveals a network issue:
+# downloading from `ftp://gcc.gnu.org/` from Travis (using passive mode) often leads to "Connection
+# timed out" error, and even when the download completed, the file is usually corrupted. This causes
+# nothing to be landed that day.
+#
+# We observed that the `gcc-4.8.5.tar.bz2` above can be downloaded successfully, so as a stability
+# improvement we try to download from the HTTPS mirror instead. Turns out this uncovered the third
+# bug: the host `gcc.gnu.org` and `cygwin.com` share the same IP, and the TLS certificate of the
+# latter host is presented to `wget`! Therefore, we choose to download from the insecure HTTP server
+# instead here.
+#
+sed -i'' 's|ftp://gcc\.gnu\.org/|http://gcc.gnu.org/|g' ./contrib/download_prerequisites
+
 ./contrib/download_prerequisites
 mkdir ../gcc-build
 cd ../gcc-build