ci: fix bash variable expansion in sed pattern

The previous sed expression had \${prefix} in a double-quoted string,
which bash was expanding to a literal prefix variable lookup. With
`set -euo pipefail` and unbound variables causing exit, the entire
script aborted right after `make install-shared`, before ldconfig
and the sanity check ran.

Use single quotes around the sed expression so bash leaves the
\${prefix} alone for sed to interpret.

Discovered via:
  scripts/ci/build-rocksdb.sh: line 57: prefix: unbound variable

scripts/ci/build-rocksdb.sh

@@ -56,7 +56,7 @@ PC_FILE="${INSTALL_PREFIX}/lib/pkgconfig/rocksdb.pc"
 if [ -f "${PC_FILE}" ]; then
     sed -i \
         -e "s|-isystem third-party/gtest-1.8.1/fused-src|-I${INSTALL_PREFIX}/include|g" \
-        -e "s|-isystem \\${prefix}/third-party/gtest-1.8.1/fused-src|-I${INSTALL_PREFIX}/include|g" \
+        -e "s|-isystem \\\${prefix}/third-party/gtest-1.8.1/fused-src|-I${INSTALL_PREFIX}/include|g" \
         "${PC_FILE}"
 fi