[lib/prep.sh] Limit calls to cargo install --list for plugins.

When checking for an installing plugins, there isn't a need to call
cargo for each plugin to check. While this is just-as-good as previous
behavior, it also limits any issues when calling a Cargo subcommand that
might fail.

Presently, there are times where a race exists and either `STDOUT` or
`STDERR` might disppear when Rustup (or potentially Cargo) is used in a
shell pipeline, leading to a program panic with `failed printing to
stdout: Broken pipe (os error 32)`. This change tries to minimize the
probability that this occurs until a more robust upstream fix has
landed.

References: rust-lang/rustup#1730
References: rust-lang/rustup#1765

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

lib/prep.sh

@@ -457,6 +457,7 @@ install_rust() {
   local rustc="$HOME/.cargo/bin/rustc"
   local cargo="$HOME/.cargo/bin/cargo"
   local rustup="$HOME/.cargo/bin/rustup"
+  local installed_plugins
 
   header "Setting up Rust"
 
@@ -482,8 +483,9 @@ install_rust() {
   "$rustup" component add rust-src | indent
   "$rustup" component add rustfmt | indent
 
+  installed_plugins="$("$cargo" install --list | grep ':$' | cut -d ' ' -f 1)"
   for plugin in cargo-watch cargo-edit cargo-outdated; do
-    if ! "$cargo" install --list | grep -q "$plugin"; then
+    if ! echo "$installed_plugins" | grep -q "^$plugin\$"; then
       info "Installing $plugin"
       "$cargo" install "$plugin" 2>&1 | indent
     fi