[Fuchsia] Fix root SSL cert path

Fuchsia stores the root SSL cert in a different location than Linux.
This commit corrects this, so that SSL connections work.

include/grpc/impl/codegen/port_platform.h

@@ -387,6 +387,7 @@
 #define GPR_POSIX_TIME 1
 #define GPR_HAS_PTHREAD_H 1
 #define GPR_GETPID_IN_UNISTD_H 1
+#define GRPC_ROOT_PEM_PATH "/config/ssl/cert.pem"
 #else
 #error "Could not auto-detect platform"
 #endif

src/core/lib/security/security_connector/ssl_utils.cc

@@ -41,11 +41,13 @@
 
 /* -- Constants. -- */
 
-#ifndef INSTALL_PREFIX
-static const char* installed_roots_path = "/usr/share/grpc/roots.pem";
-#else
+#if defined(GRPC_ROOT_PEM_PATH)
+static const char* installed_roots_path = GRPC_ROOT_PEM_PATH;
+#elif defined(INSTALL_PREFIX)
 static const char* installed_roots_path =
-    INSTALL_PREFIX "/share/grpc/roots.pem";
+    INSTALL_PREFIX "/usr/share/grpc/roots.pem";
+#else
+static const char* installed_roots_path = "/usr/share/grpc/roots.pem";
 #endif
 
 #ifndef TSI_OPENSSL_ALPN_SUPPORT