Repository: nvm
Issue: No npm prefix set, but nvm requires 'nvm use --delete-prefix'
Labels: feature requests,pull request wanted

- Operating system and version:
<details>
  <!-- do not delete the following blank line -->

```sh
$ lsb_release -a
LSB Version:	1.4
Distributor ID:	Arch
Description:	Arch Linux
Release:	rolling
Codename:	n/a
```
</details>


- `nvm debug` output:
<details>
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```sh
$ nvm debug
nvm --version: v0.34.0
$SHELL: /bin/bash
$SHLVL: 2
$HOME: /home/paulk/
$NVM_DIR: '$HOME/.nvm'
$PATH: /usr/local/sbin:/usr/local/bin:/usr/bin:/opt/android-sdk/platform-tools:/usr/lib/jvm/default/bin:/usr/lib32/jvm/default/bin:/opt/ti/msp-flasher:/opt/ti/mspgcc/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:$HOME/bin/:$HOME.gem/ruby/2.3.0/bin:/opt/android-sdk/platform-tools/:$HOME.bin:$HOMEbin/gyb:
$PREFIX: ''
$NPM_CONFIG_PREFIX: ''
$NVM_NODEJS_ORG_MIRROR: ''
$NVM_IOJS_ORG_MIRROR: ''
shell version: 'GNU bash, version 5.0.7(1)-release (x86_64-pc-linux-gnu)'
uname -a: 'Linux 5.1.15-zen1-1-zen #1 ZEN SMP PREEMPT Tue Jun 25 04:49:28 UTC 2019 x86_64 GNU/Linux'
OS version: Arch Linux  ()
curl: /usr/bin/curl, curl 7.65.3 (x86_64-pc-linux-gnu) libcurl/7.65.3 OpenSSL/1.1.1c zlib/1.2.11 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh2/1.8.2 nghttp2/1.36.0
wget: /usr/bin/wget, GNU Wget 1.20.3 built on linux-gnu.
git: /usr/bin/git, git version 2.22.0
grep: /usr/bin/grep (grep --color=auto), grep (GNU grep) 3.3
awk: /usr/bin/awk, GNU Awk 4.2.1, API: 2.0 (GNU MPFR 4.0.2, GNU MP 6.1.2)
sed: /usr/bin/sed, sed (GNU sed) 4.7
cut: /usr/bin/cut, cut (GNU coreutils) 8.31
basename: /usr/bin/basename, basename (GNU coreutils) 8.31
rm: /usr/bin/rm, rm (GNU coreutils) 8.31
mkdir: /usr/bin/mkdir, mkdir (GNU coreutils) 8.31
xargs: /usr/bin/xargs, xargs (GNU findutils) 4.6.0
nvm current: system
which node: /usr/bin/node
which iojs: which: no iojs in (/usr/local/sbin:/usr/local/bin:/usr/bin:/opt/android-sdk/platform-tools:/usr/lib/jvm/default/bin:/usr/lib32/jvm/default/bin:/opt/ti/msp-flasher:/opt/ti/mspgcc/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:$HOME/bin/:$HOME.gem/ruby/2.3.0/bin:/opt/android-sdk/platform-tools/:$HOME.bin:$HOMEbin/gyb:)
which npm: /usr/bin/npm
npm config get prefix: /usr
npm root -g: /usr/lib/node_modules
```
</details>

- `nvm ls` output:
<details>
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```sh
$ nvm ls
        v8.16.0
       v10.16.2
->       system
node -> stable (-> v10.16.2) (default)
stable -> 10.16 (-> v10.16.2) (default)
iojs -> N/A (default)
unstable -> N/A (default)
lts/* -> lts/dubnium (-> v10.16.2)
lts/argon -> v4.9.1 (-> N/A)
lts/boron -> v6.17.1 (-> N/A)
lts/carbon -> v8.16.0
lts/dubnium -> v10.16.2

</details>

- How did you install `nvm`? (e.g. install script in readme, Homebrew):
nvm installed from https://aur.archlinux.org/packages/nvm/
My ~/.bashrc ends with:
<details>

  <!-- do not delete the following blank line -->

```sh
export PATH="$PATH:$(yarn global bin)"
export NVM_DIR="$HOME/.nvm"
source /usr/share/nvm/init-nvm.sh
```
</details>

- What steps did you perform?
Anytime I run `nvm use {NODE_VERSION}` it complains 
<details>
  <!-- do not delete the following blank line -->

```sh
nvm is not compatible with the npm config "prefix" option: currently set to "/home/paulk/.nvm/versions/node/v{NODE_VERSION}"
Run `npm config delete prefix` or `nvm use --delete-prefix v{NODE_VERSION}` to unset it.
```
</details>

If I set a default alias, then the error message appears anytime I open a shell. NVM seems to work just fine if I use `nvm use --delete-prefix` every time I try to load a different NVM version.

- What did you expect to happen?
I expected nvm to silently switch to the specified node. I'd like to be able to set a default prefix.

- Is there anything in any of your profile files (`.bashrc`, `.bash_profile`, `.zshrc`, etc) that modifies the `PATH`?
Yes, but it's all before the lines that load nvm.

My ~/.npmrc contains only the `local-address=10.1.2.123` option.